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.
