The Drive Cycle and Emissions Readiness Monitors are methods used by the Engine Computer or PCM to determine whether an Emissions System repair was properly performed. A Drive Cycle is a special test drive that duplicates a person starting their car and making a short freeway trip, as if they were driving to work. While this special test drive or 'Drive Cycle' is occurring, the Engine Computer runs little tests or Readiness Monitors that check to see if the Emissions System is working properly.

When a vehicle has an Emissions System problem, it almost always triggers a Check Engine or Service Engine Soon Light. This signals that an Emission System problem and fault code has been recorded in the Power Train Control Module or PCM. This Emissions System problem, indicated by the fault code, must now be accurately diagnosed and repaired. 

After the proper repair has been completed and the fault code cleared, the Power Train Control Module or PCM will run a series of self-tests during a special test drive called a Drive Cycle.  These tests are to determine whether or not the repair actually corrected the problem and whether the various Emissions Systems are 'Ready' to be trusted. If the Emission Systems are 'Ready' then they are put back in to the service of minimizing the Emissions released in to the atmosphere from the vehicle's operation. This process is how the term 'Emissions Readiness Monitors' was derived. This process was designed to prevent a vehicle from slipping through an Emissions Test with a known Emissions problem. Until 1996, a common tactic was to turn off the Check Engine Light just by clearing the code just before an Emissions Test, with out performing the proper repair. The Drive Cycle and Emissions Readiness Monitors have, for the most part, stopped this unethical tactic.

Why is there a Check Engine Light in the first place?

Every vehicle manufactured to be sold in the U.S. has to pass an Environmental Protection Agency (EPA) test procedure called the Federal Test Procedure. This sets the acceptable limits of wear and/or failure for the emission-control system—i.e., what conditions ultimately cause a Check Engine Light to illuminate. These standards are closely regulated. If the emission-control system is faulty and the vehicle is polluting the air, the Check Engine Light illuminates to alert the driver of this condition. (A vehicle in this condition would fail an emissions inspection or smog check.)

Don't confuse the Check Engine Light with the maintenance or service light. These lights illuminate when a routine service is due. They are usually triggered by mileage, gallons of gasoline consumed, or some other type of vehicle-use measurement.

What is a Drive Cycle?

A Drive Cycle is basically a special type of test drive that resembles a person going out and driving their vehicle after it has been sitting over night. First they cold start their engine and it warms up a bit while they buckle their seat belts, select the radio station or CD they want, adjust the volume and let the windows defrost a little. Then they do some stop and go driving through city streets at city driving speeds that range from 25-35 mph. Then they pull on to a freeway entrance and accelerate briskly at 1/2 throttle or more to match the speed of the other drivers as the they enter the flow of the freeway traffic. Then the vehicle is driven at a steady, conservative speed of between 55 and 60 mph for at least 4-5 miles. After this 4-5 mile freeway cruise, the vehicle pulls off the freeway and and is driven a bit more in stop and go city driving conditions and is then pulled into a parking place where the engine idles for 20-30 seconds and is then turned off. On many vehicles, 1 of these 'Drive Cycles' is sufficient to set all or most of the Readiness Monitors. Other vehicles require that this entire process be repeated on 2 or more successive days.

What are the Readiness Monitors?

The Emission Control System has several sub-systems that are Continuously or Non-Continuously Monitored. This is to control the Emissions released by the vehicle when it is being operated and even while it is parked. 

There are 3 sub-systems that are Continuously Monitored. This means that while the vehicle is being operated, the Power Train Control Module is Monitoring and evaluating the performance and accuracy of these sub-system 100% of the time. They are as follows:

• The Continuous Component Monitor is the method used by the Power Train Control Module to determine the accuracy of all the Engine and Transmission Sensors and Outputs, such as the Throttle Position Sensor, the Mass Air Flow Sensor, the Fuel Injectors, the Alternator, the Transmission Speed Sensors and Shift Solenoids etc. When ever the the Power Train Control Module notices any type of anomaly in their performance, either electrical or mechanical, the Check Engine Light is illuminated and a trouble code is stored in the memory of 1 or more onboard Computers. 
• The Misfire Monitor is the method used by the Power Train Control Module to determine whether the engine is malfunctioning to the point where it is releasing excess pollutants in to the air. The PCM keeps track of the Crankshaft speed after the combustion of each cylinder and when it varies by just 2%, the Check Engine Light is illuminated and 1 or more MisFire codes are stored in the memory of the PCM. If the Misfire causes the Crank Shaft Speed to vary by more than 10%, the Check Engine Light will blink 2 times a second to warn the vehicle operator that a severe, Catalytic Converter damaging Misfire condition is occurring. 
• The Fuel System Monitor is the method used by the Power Train Control Module to ensure that the Air and Fuel ratio being burned in the combustion chambers is as close to 14.7 parts Air to 1 part Fuel as possible. This optimizes the performance and fuel efficiency of the engine while releasing the least amount of harmful Emissions. It also maximizes the performance or 'after burning' effect of the Catalytic Converter.

There are 5 or more Non-Continuously Monitored sub-systems. This means that while the vehicle is being operated and even while it is parked, the Power train Control Module is testing the performance of these sub-systems during very specific operating conditions. These conditions may only occur once every 24 hours, such as after a vehicle has been parked for at least 8 or more hours and is then performs what is termed a 'stone cold' start up cycle.

• The Oxygen Sensor Heater Monitor checks the operation of the electrical heating element that enables the Oxygen Sensor to keep its ideal operating temperature of 600 degrees Fahrenheit. When most vehicles idle, the Oxygen Sensor tends to cool off and therefore lose its ability to accurately read the Oxygen content in the exhaust gases. The heating element, which is much like the heating element inside of a hair dryer, is energized when ever most vehicles are motionless and the engine is idling. 
• The Oxygen Sensor Monitor checks the performance of the Oxygen Sensor's ability to accurately and very quickly decipher the Oxygen content in the exhaust gases as they exit the combustion process. On most vehicles, the Oxygen Sensor Heater Monitor most run successfully before the Oxygen Sensor Monitor is tested or run.
• The Catalytic Converter Monitor checks the Emissions reducing ability of the 1 or more Converters installed in the vehicle. It does this by using a special, dedicated Oxygen Sensor that is installed at or near the outlet of each monitored Converter. On most vehicles, both the Oxygen Sensor Heater Monitor and the Oxygen Sensor Monitor must run successfully before the Catalytic Converter Monitor will run.
• The EGR Monitor checks the operation of the EGR System in a variety of operating phases. Many vehicles use a 2 tiered EGR Monitor that checks the EGR System at freeway speeds, usually during deceleration, and during stop and go city driving conditions and speeds. Often the Oxygen Sensor Heater Monitor must successfully run before the EGR Monitor will run.
• The EVAP Monitor checks the vehicle EVAP System for proper performance by running leak and performance checks. This is to ensure that there are no holes or cracks in the fuel storage system that are larger than the tip of a pin ( 0.020" - 0.040" ). The EVAP Monitor also checks the ability of the EVAP System to recycle any stored fuel vapors back in to the combustion process during vehicle operation. Often, the EVAP Monitor will only run after the vehicle has been shut off and sitting for 8 hours or more. On some vehicles, both the Oxygen Sensor Heater Monitor and the Oxygen Sensor Monitor must have been run successfully before the EVAP Monitor will run.
• The Secondary Air System Monitor checks that the various components and functions of the Secondary Air System are performing properly. Most Secondary Air Systems run when the vehicle has its first 'cold start' after sitting over night and rarely run unless the vehicle has been sitting for an extended time period in a cool or cold weather environment. The Oxygen Sensor Heater and Oxygen Sensor Monitor must run and be 'Ready' before the Secondary Air System Monitor will run, however the system will operate whenever the start up parameters are met.
• The Air Conditioning Monitor is being added to more vehicles as Automatic Climate Control becomes the norm. This Monitor verifies that the Engine idle remains smooth and does not misfire, releasing harmful Emissions, any time the A/C Compressor is energized. This is because the operation of the A/C Compressor adds a 20+ horsepower load to the Engine. By checking the operational line pressures, the Air Conditioning Monitor can also determine if the A/C System is leaking and releasing harmful, Ozone depleting Chlorofluorocarbons in to the atmosphere. The Air Conditioning Monitor runs after most if not all of the other  Monitors have run successfully and are 'Ready'.
  
• The Heated Catalyst Monitor checks the condition and operation of the large heating elements that heat the very large Catalytic Converters on high performance vehicles during cold start conditions. Very often these are exotic vehicles with the most powerful 10-12 Cylinder Engines that burn tremendous amounts of fuel under hard acceleration. The Heated Catalytic Converter also checks the extra Battery and larger Alternator that is required to energize the heavy duty heating elements inside the Heated Converters.

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