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Drive Cycle and Emissions Readiness Monitors

The Drive Cycle and Emissions Readiness Monitors are methods used by the powertrain control module (PCM) to determine whether an emissions system repair was properly performed. A Drive Cycle is a special test drive that duplicates the scenario of a person starting her car and making a short freeway trip, as if she were driving to work. While the Drive Cycle test is going, the engine computer runs little tests or "readiness monitors" 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 powertrain control module (PCM). The 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 PCM will run a series of self-tests to determine whether or not the repair actually corrected the problem and if the various emissions systems are running properly. If they are, they can now properly minimize the emissions released into the atmosphere from the vehicle's operation.

This process was designed to prevent a vehicle from slipping through an emissions test with a known problem. Until 1996, a common tactic was to turn off the Check Engine Light by clearing the code just before an emissions test, without 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 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 will illuminate. (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 replicates the conditions of a person going out and driving his vehicle after it has been sitting overnight. First, he cold starts his engine and it warms up a bit while he is buckling his seat belt, selecting the radio station or CD he wants and adjusting the volume, and letting the windows defrost a little. The car is then driven through stop and go scenarios through city streets at speeds of 25 to 35 mph. Then, the car enters the freeway and accelerates briskly at half 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 four to five miles.

After this, the vehicle pulls off the freeway and is driven a bit more in city driving conditions before pulling into a parking place where the engine idles for 20 to 30 seconds before being turned off. On many vehicles, one of these Drive Cycles is sufficient enough to set all or most of the Readiness Monitors. Other vehicles require that this entire process be repeated on two or more successive days.

What Are the Readiness Monitors?

The emission control system has several sub-systems that are continuously or non-continuously monitored in order to control the emissions released by the vehicle when it is being operated or parked.

Continuously Monitored Sub-Systems

There are three sub-systems that are continuously monitored. While the vehicle is being operated, the PCM is monitoring and evaluating the performance and accuracy of these sub-system 100 percent of the time. The sub-systems are:

  • Continuous Component Monitor
    The continuous component monitor is the method used by the PCM to determine the accuracy of all of 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 the shift solenoids, etc. Whenever the the PCM 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 one or more onboard computers.
  • Misfire Monitor
    The misfire monitor is the method used by the PCM to determine whether the engine is malfunctioning to the point where it is releasing excess pollutants into the air. The PCM keeps track of the crankshaft speed after the combustion of each cylinder. When it varies by just 2 percent, the Check Engine Light is illuminated and one or more misfire codes are stored in the memory of the PCM. If the misfire causes the crankshaft speed to vary by more than 10 percent, the Check Engine Light will blink two times a second to warn the vehicle operator that a severe, catalytic converter-damaging misfire condition is occurring. 
  • Fuel System Monitor
    The fuel system monitor is the method used by the PCM to ensure that the air-to-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.

Non-Continuously Monitored Sub-Systems

There are five or more non-continuously monitored sub-systems. While the vehicle is being operated, and even while it is parked, the PCM is testing the performance of these sub-systems during very specific operating conditions. These conditions may only occur once every 24 hours, such as "cold starting" after a vehicle has been parked for at least eight or more hours. These sub-systems are:

  • Oxygen Sensor Heater Monitor
  • Oxygen Sensor Monitor
  • Catalytic Converter Monitor
  • EGR Monitor
  • EVAP Monitor
  • Secondary Air System Monitor
  • Air Conditioning Monitor
  • Heated Catalyst Monitor
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