Auto Systems and Repair: How to Understand Trouble Codes
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
1. 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 Converters could 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 overfueling the engine for a reason that is not detectable by any other on board diagnostic capability.