How to Understand Diagnostic Trouble Codes

Since modern vehicles are constantly monitoring their own operation through complex computer systems, trouble/OBD codes are becoming an increasingly common part of car repair. When the vehicle's computer system senses a problem, a trouble code is produced, which often appears on the dashboard as a Check Engine Light (CEL), as a part of the On Board Diagnostics system.

Since modern vehicles are constantly monitoring their own operation through complex computer systems, trouble/OBD codes are becoming an increasingly common part of car repair. When the vehicle's computer system senses a problem, a trouble code is produced, which often appears on the dashboard as a Check Engine Light (CEL), as a part of the On Board Diagnostics system.

There are two versions of On-Board Diagnostics (OBD):

OBD I (Vehicle model years 1988–1995)

In OBD I systems, when a fault condition occurs, the CEL stays on until the condition is resolved. This inhibits the driver from ignoring the problem. If the driver continues to ignore the problem, the vehicle will fail the next state-mandated smog check, which prevents the vehicle from being registered or "legally" sold. Some private sellers or dealers may attempt to sell a vehicle that has a code setting condition that is polluting the environment—this is illegal.

OBD II (Vehicle model years 1996–present)

OBD II is essentially OBD I with a new set of monitoring systems built on top. The monitors are bundles of software with increased testing capacity that either cycle constantly or when the system in question is in operational mode.

Included in this article are descriptions of:

  1. Continuous Component Monitor
    The Continuous Component Monitor is an updated version of OBD II and primarily focuses on pollution control. From the time the key is inserted into the ignition, the computer system boots and starts running self checks on all the powertrain components. This section focuses on cranking voltage, the Misfire Monitor system, and Fuel Trim Adaptation.

  2. Non-Continuous Component Monitor
    The Non-Continuous Component Monitors can vary between vehicle engine management systems, but there are several that are common to most vehicles. The term "non-continuous" means that the system and components are checked only under specific operating conditions. For example, in hot climates, there aren't any cold start conditions, so "stone-cold" startup monitoring will not occur at all in those places. This section focuses on common monitors, including the Oxygen Heater Monitor, Oxygen Sensor Monitor, and Catalyst Monitor.

  3. Basic Sections of an OBD II Catalytic Converter
    The next section focuses on the basic parts of an OBD II Catalytic Converter and the EVAP and EGR Monitors.

  4. Additional Monitors
    The last section of the article focuses on less common monitors like the Secondary Air System Monitor, Catalytic Heater Monitor, and Air Conditioning Monitor. It also has a section on the Mode 6 OBD II Data Stream for the Non-Continuous Monitors.

OBD I (On Board Diagnostics Generation One)

This was the first type of Check Engine Light trouble code setting system. Up until 1988, most codes would generally disappear when the engine or key was turned off. The trouble code would not "set" in the memory of the Engine Control Module (ECM). In 1988, the more capable 32-bit processor system was starting to be introduced into automotive engine management computers. This allowed for a more comprehensive "on board" or "self-diagnosing" system, as well as the ability for trouble codes to be set in the memory.

As the engine management systems used more sensors and actuators, it became necessary to create additional monitoring methods. The sensors had to be grouped into systems and sub-systems to accurately keep track of their behavior. The more complex actuators displayed vastly different failure profiles than the sensors. The engine computer began to monitor itself to ensure that it was receiving proper voltages and grounds, and that the on-off switching parts of the computer (drivers) were not being overly taxed by a worn or shorting actuator.

It became clear that types of fault or code setting conditions existed. Some faults were "hard" faults, which meant that they were always present. Then, there were "intermittent" faults, which meant that the code set only under certain conditions, such as temperature range, vehicle speed, vehicle load, and vehicle time of operation, among others. With the newly capable engine computer and the OBD I system, a code set under intermittent conditions would stay in the diagnostic memory of the vehicle, making it possible for a technician to diagnose and repair the problem, even if the fault was not occurring at the present time.

When a fault occurred, it was now possible to employ a backup program or "limp home" mode in the engine management system. This allows the computer to employ a decreased capability program to help the vehicle get home, so the vehicle operator is not left stranded on a road way.

The introduction of the OBD system also meant that the transmission was now being included in the emissions system. Either the engine computer or, in some cases, an additional dedicated transmission module, was now linked to the primary engine control module. This meant that the OBD I system software had to track the proper operation of two multi-dimensional components that needed to work in harmony. The EPA found that a poorly functioning transmission could cause the engine to consume excessive fuel or cause excessive NOx pollution from the engine due to improper torque load. OBD I was a breakthrough in the management of these conditions since it had the capability to manage more than one computer.

However, as time passed, it became clear that OBD I was still too limited in its scope of managing the on board vehicle diagnostics. OBD I mainly concerns itself with sensors and actuators going out of range or into complete failure. By the early to mid 1990s, the sensors and devices grew to the point where system and sub-system monitors had to be created that ran only once a day or once every time the vehicle was stone-cold started. Even the time required to warm up the vehicle became scrutinized and fault codes were implemented to notify the driver if the vehicle was warming up improperly. The EPA found that most of the air pollution caused by vehicles occurs during the warm up cycle of operation. The faster a vehicle warms up, the cleaner it runs.

By the mid 1990s, some of the more complex or luxury class vehicles used dozens of sensors and actuators deployed in the transmission and the engine. By the early 1990s, the automatic transmission was an integral part of the emissions system. The EPA found that poorly functioning transmissions caused vehicles to consume more fuel and produce very high levels of NOx (a toxic gas) if the proper gear ratio could not be achieved. Computer networks began to control the transmission and engine systems with safety systems such as anti-lock brake systems (ABS), supplemental restraint systems (airbags), and traction control for safer driving on snowy/icy roads. The computer networks allowed for all of these systems to be synchronized with each other. If there was a fault in the engine management or transmission system, the other safety systems could be disabled and a warning light illuminated to warn the vehicle operator that one (or more) safety systems were disabled.

Another key limitation of OBD I—vehicle wear—became clear in the 1990s. The EPA found that as a vehicle was used over time, the various engine management systems did not accurately account for the mechanical degradation of the engine and its internal components. Though a sensor or device still worked properly, it could be operating at the limit of its range. Major fuel system sensors and devices, such as oxygen sensors and fuel injectors, were at their functional limit, yet still had not set any codes. This meant that under certain operating conditions, the vehicle was polluting the air and not setting a Check Engine Light code.

It also became clear that one of the major pollution issues—engine misfires—was caused either by ignition component wear, mechanical wear, or fuel system problems. Misfires cause raw HCs or raw gasoline to be released in the air. Under OBD I, an engine could have severe misfires and be in a "gross polluting" operational condition, but not be operating on all cylinders and not be setting a Check Engine Light code. The EPA mandated a reliable solution to this unacceptable operating condition—the "engine misfire" monitoring system—which led to OBD II (On Board Diagnostics Generation Two).

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OBD II (On Board Diagnostics Generation Two)

OBD II is essentially OBD I with a new set of monitoring systems built on top. The monitors are bundles of software with increased testing capacity that either cycle constantly or when the system in question is in operational mode.

The monitoring systems are divided into two distinct categories:

  1. Continuous monitors
  2. Non-continuous monitors (skip to this section)

Continuous Component Monitor

The Continuous Component Monitor is an updated version of OBD II. From the time the key is inserted into the ignition, the computer system boots and starts running self checks on all the powertrain components.

Among the first items monitored during startup are barometric pressure and system voltage while the vehicle is being cranked. If the vehicle cranking voltage falls below a threshold value—usually around 9.8 volts—the rest of system is put into a compromised mode because the accuracy of the electrically energized sensors and actuators cannot be trusted.

If the cranking voltage is sufficiently above the correct threshold, then the rest of the continuous component monitoring system will engage—all of the powertrain sensors and actuators will be voltage- and current draw-checked several times per second. If any of the sensors or actuators are out of spec, then a Check Engine Light will illuminate, signaling that a code setting/fault event has occurred.

The newest addition to the CCM (Continuous Component Monitor) is the Misfire Monitor system. This system became critical as the EPA gathered data on the number of people with grossly polluting vehicles, primarily due to misfiring cylinders. People would often drive for up to two years (until their next emissions check) with a vehicle emitting literally thousands of HCs parts per million—this is equivalent to a single vehicle putting out the same emissions as 2,000 to 3,000 vehicles.

It was also discovered that some of the emission control components were destroyed by the raw, unburned fuel or (HCs) being pumped through the system. The oxygen sensor(s) could be ruined and the Catalytic Converterscould be ruined or even melted from overheating. The mechanical base engine suffered damage by the raw fuel from the misfires getting into the oil and significantly diluting it. Fuel-saturated engine oil became a known problem, though it had existed for years, under the radar.

The Misfire Monitor (MM) was designed to directly address the problem of Hydro Carbon pollution. The MM is a high level software program that constantly counts the average time (in milliseconds) between firings of every cylinder. When the vehicle is new, the MM goes through an adaptation period where it learns the "normal" time between each cylinder-firing event, in the correct sequence. Once the normal time is learned, the MM keeps track of any deviations from the norm, whether it's too long or too short. If a 2 percent change is noticed, the MM logs this in its misfire count table. If the 2 percent variation persists for a long enough time (determined by a factory EPA-approved spec), a Check Engine light will illuminate. The fault code may be for an individual cylinder, a group of cylinders, or for all the cylinders at once, usually called a "random misfire" fault. These types of misfires are sometimes so subtle that only the Engine Computer Unit (ECU) can perceive them. Other types of misfires are much more noticeable.

If the variation between firing events is 10 percent or more, the CEL not only illuminates, but will blink multiple times a second. This is to inform the vehicle operator that a Catalytic Converter-damaging misfire is occurring and that the Catalytic Converter (CAT) may overheat to the point where it can light the vehicle on fire.

Compared to the OBD I versions, the newer OBD II converters are very sensitive. A new rare metal—Cerium—was introduced that stores and releases oxygen, depending on the oxygen content of the current exhaust flow. Misfires can destroy this part of Catalytic Converter in seconds. Many of the high-end vehicles must have their CAT replaced after a blinking Check Engine Light event. This can cost up to and above $3,000, in addition to the cost of the misfire repair(s).

Fuel Trim Adaptation is the last part of the Continuous Component Monitor. This was present in the OBD I system on some of the higher-priced vehicles. "Fuel trim" is a bit similar to the adaptive quality of the MM. When the vehicle is brand new, the fuel injector "on times" for each cylinder are learned for all driving conditions—i.e. idle, light load, highway speed, hard acceleration, etc. The change of the "on time" is triggered by exhaust oxygen content due to engine temperature, load speed, etc. As an engine and powertrain wear under normal driving conditions, the injector "on time" is slightly increased to compensate for this wear and growing inefficiency. A factory and EPA approved spec is allowed—usually below a total change of +/- 15 percent. Once the fuel trim has to adapt to more than 15 percent increase or decrease, a Check Engine Light will illuminate. Depending on the increase or decrease in fuel trim, three major pollutants will be emitted.

If the fuel trim has to increase too much, NOx and HC pollution can/will occur. If the fuel trim has to decrease or subtract injector "on time" too much, then CO pollution will occur since the powertrain management system is over-fueling the engine for a reason that is not detectable by any other on board diagnostic capability.

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Non-Continuous Component Monitor

The Non-Continuous Component Monitors can vary between vehicle engine management systems, but there are several that are common to most vehicles. The term "non-continuous" means that the system and components are checked only under specific operating conditions. Some of the monitoring will occur only once a day. For example, in very hot climates, like those in Arizona and New Mexico, the needed cold start conditions will not be attained for an entire summer, so "stone-cold" startup monitoring will not occur at all during that time.

The Oxygen Heater Monitor is common to all OBD II vehicles. This a multi-factored current/amperage flow test of the heating element common to all modern oxygen sensors and Air Fuel Ratio sensors—AFR is a highly responsive type of oxygen sensor common on most low emission vehicles (LEVs). Usually, as the vehicle is cranked, the Oxygen Heater Monitor checks the vehicle's system voltage. If it is within spec, then soon after startup or the next shut down, the OBD II system will run a series of tests on the heater elements of the vehicle's oxygen sensors. If this monitor passes, then many of the other non-continuous monitors will run. If this monitor fails, then most of the other non-continuous monitoring will be suspended or ignored. Accurate oxygen sensor performance is vital to proper engine emissions management.

The Oxygen Sensor Monitor is common to all ODB II vehicles. After the Oxygen Heater element passes its monitor, the Oxygen Monitor begins to look for the proper conditions to be run. There are a few common conditions that are required on all vehicles in order for the Oxygen Sensor Monitor to run. First, there must not be any misfire codes. Misfire conditions cause erratic oxygen exhaust content, so it is not possible for the monitor to run any accurate oxygen sensor tests. There must not be any fuel trim codes or any excessively lean or rich compensation percentages. This too will skew the oxygen sensor tests, so fuel trim issues will suspend the Oxygen Sensor Monitor.

If the above basic criteria are met, the ODB II system will begin running oxygen sensor tests under a variety of conditions. Some of the most common operating conditions are hard acceleration, highway cruise speed, prolonged idle that is at least 60 seconds, and others depending on the type of vehicle in question and the number of oxygen sensors on board. Some high performance vehicles have a front oxygen sensor for each cylinder and a rear oxygen sensor for the rear of each Catalytic Converter, meaning that some vehicles have up to and above sixteen oxygen sensors.

Since the Catalyst Monitor needs to be sure that the oxygen sensors are reading the exhaust gas oxygen content accurately, the Catalyst Monitor runs after all the oxygen monitors have run. The Catalyst Monitor is testing the ability of the CAT to store oxygen in its Cerium bed. The purpose of the Cerium is to optimize the efficiency of all the sections of the Catalytic Converter.

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Basic sections of a catalytic converter

There are three basic sections of an OBD II catalytic converter.

  1. The first section is called the reduction section; it reduces NOx down to oxygen and nitrogen.

  2. The Cerium section of the Catalytic Converter stores oxygen, where it is used in the oxidizing section, converting CO and raw HCs into CO2 and 02. This section is designed to maintain a predetermined threshold of oxygen so if the Catalytic Converter exhaust oxygen content gets too low, the Cerium releases it. If it gets too high, it stores it, thereby maintaining an ideal or Stoichiometric ratio of exhaust gases. (Stoichiometric is an ideal air to fuel ratio that maintains the cleanest balance of emissions "after burning" in the Catalytic Converter. Numerically, it is 14.7 parts air to 1 part fuel.)

  3. The Catalytic Converter Monitor uses the voltage and response times of the front oxygen sensors and rear oxygen sensors to determine the "scrubbing" ability of the CAT. The monitor watches the behavior of the front oxygen sensor in relation to the rear oxygen sensor on each bank or line of engine cylinders. The front oxygen sensor is usually near the exhaust port, in or near the exhaust manifold, but before the inlet of the CAT. The rear oxygen sensor is near or on the rear outlet of the CAT. The CAT Monitor wants to see a change in behavior from the front oxygen sensor versus the rear oxygen sensor. Depending on the change in electrical signals from the front to rear sensors, as the exhaust gases are converted from flowing through the CAT, the Catalytic Monitor determines that the converter is effective and within specified tolerance, as set by the manufacturer and in accordance with EPA guidelines.

The Catalytic Monitor is mostly run at highway speeds, usually at 55–60 MPH for two to four minutes. On some LEV or ULEV (low emission or ultra low emission) vehicles, the CAT Monitor is run at several speeds, including city speeds, idle, and even hard acceleration. This is why some vehicles need very sophisticated Air Fuel Ratio-type oxygen sensors that can cost as much as $500 each. These sensors help keep the vehicle in a very tight Air/Fuel window.

The Evaporative Monitor is also common to most vehicles. Up to 20 percent of a vehicle's of air pollution is caused by leaking raw fuel vapor from the vehicle fuel storage system—the fuel cap, filler neck, fuel tank, fuel lines, fuel vapor canister, and included valves and vacuum lines.

The EVAP monitor has some specific enabling criteria. The fuel tank must be between 15 and 85 percent full. This is because the monitoring algorithms depend on a consistent (within a range) vapor pressure. If the fuel tank level is too low or too high, the fuel tank pressure sensor data readings cannot be trusted, so the EVAP monitor will not run. The other enabling criteria are that the vehicle must be "stone-cold," meaning that the engine has been off and the key out of the ignition (many vehicles are not really "off" until the ignition key is out) for at least eight hours. This allows the engine to be within 1 to 5 degrees of the ambient air temperature, a cooling process that takes eight hours or more. Some vehicles actually run the EVAP Monitor while the engine is off. This is done by a separate, tiny computer that is dedicated to running the EVAP test. It will come on after the vehicle has been "off" for the appropriate amount of time.

The EVAP Monitor is basically a leakage test. A vacuum condition is created in the fuel tank and vapor canister—then the vacuum data is recorded. The OBD II software can detect how large a leak there is in the system. Most 2000 and newer vehicles fail the EVAP monitor if there is a .020" leak or larger. This is about half the width of a head of a pin!

Many vehicles have an EGR system monitor. The purpose of the Exhaust Gas Recirculation (EGR) system is to literally recirculate inert exhaust gas back through the combustion chambers in order to cool the peak firing temperatures to below 2500º F. This below 2500º condition prevents the formation of nitrogen oxides (NOx), which are poisonous gases. When the heat is 2500º or above, the oxygen and nitrogen molecules will actually bond and form various NOx compounds.

Most EGR systems are capable of recirculating up to 10 percent of the exhaust gas back through combustion. The EGR Monitor usually is enabled during light cruise conditions after the vehicle is fully warmed up. The EGR Monitor is known for being a two-trip monitor, meaning it must fail two times in a row on two successive startups and drives in order to set a code. The EGR Monitor also checks for maximum flow capacity during highway deceleration. This is to verify that the system is capable of delivering the 10 percent flow. Since it is done during deceleration, the driver will not be able to detect any unusual engine performance. If the "maximum flow" check was done at almost any other time, the engine would run rough and hesitate.

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Additional monitors

Secondary Air System Monitor

The Secondary Air System is a form of air injection into the exhaust system in order to "after burn" CO and HCs. Most of the modern air injection systems run only during cold or cool start up and most vehicles have some sort of air pumping system to push fresh air (oxygen) to help burn the dirtier exhaust during cold/cool engine operation. When the vehicle is warmed up, the exhaust system and CAT are much more effective and the additional, or secondary, air is no longer needed to bring the tail pipe emissions into compliance.

Catalytic Heater Monitor

The Catalytic Heater Monitor is similar to the Oxygen Sensor Heater Monitor, only on a much larger scale. These vehicles have a large heater element inside the Catalytic Converter(s) to heat the CAT on cold/cool start up and, in some cases, keep the CAT hot during long idle durations. Usually there are up to two dedicated batteries to heat the CAT(s) because the current is in the hundreds of amps. The normal vehicle charging system and battery could not handle this enormous electrical load. Almost all of the vehicles that have these types of electrically heated converters are powerful, exotic 10 to 12 cylinder vehicles with horsepower ratings of 350 and more. There is so much fuel being burned that the only way to keep them in compliance is to have extreme emissions equipment.

Air Conditioning Monitor

A few vehicles have an Air Conditioning Monitor, though it's not very common. There are few reasons why this monitor exists. It was discovered that some vehicles were polluting when the air conditioning system was turned on due to the 20+ horsepower load put on the engine from the air conditioning compressor. Vehicle manufacturers actually had to change the engine management to keep the engine idle smooth. They did this by making the engine run richer to the point where it was polluting when the A/C was engaged. Another reason was to check for CFC pollution. If the A/C system did not hold a certain level of pressure, then it was known that the refrigerant had leaked out into the environment, which hurts the ozone layer.

Most of the vehicles with an Air Conditioning Monitor have the A/C system on all the time. The vehicle operator has to manually turn off the A/C system to allow the A/C Monitor to run.

Mode 6 OBD II Data Stream for the Non-Continuous Monitors

Briefly, this is a way for technicians to "witness" the actual scientific test output data of the non-continuous monitor group. Each monitor has many tests, which are run in several sequences. Their output is usually in a hexadecimal numeric data stream. If a technician wants to drill down to the finer points of the cause of a monitor failure, there are ways to access the data conversions from the manufacturer to see what part of the system or component is failing and in what stage.

For example, the oxygen sensor may be voltage-high and low-range responsive, but it may be a few milliseconds slow for a particular OBD II system. The Mode 6 data stream would show this. After many, many miles of driving and cold starts, a monitor may not set a trouble code—the monitor is considered "hung." The Mode 6 data can bring to light what part of the monitor testing is close to, but not quite, failing. The monitor will run almost endlessly without passing or failing, sometimes for months and hundreds, even thousands of miles. This can be a huge problem for someone trying who needs his vehicle to pass an emissions test and the vehicle keeps failing the OBD II functional portion due to a "not ready" monitor(s) status.

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i plugged a diagnostic device to my truck but it tells me that there are no codes avail(no data) a friend told me that many mechanics erase those codes when they test for smog that way u never know whats wrong and u end up having to pay them to diagnose your car how do i get those codes back?