A drive cycle is a special test drive that duplicates the scenario of a person starting their car and making a short freeway trip, as if the person was driving to work. While the drive cycle test is going, the engine computer runs little tests to see if the emissions system is working properly. If during the test the computer senses a 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 faulty code must now be accurately diagnosed and repaired.
Once the problem has been corrected and the fault code cleared out of the system the computer 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 correctly. Once the computer sees that there are no problems then each monitor will be cleared.
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.
Still have questions about emissions readiness monitors? Contact The Auto Analyst, your one stop auto repair facility, at (530) 621-4591 or visit our website at www.theautoanalyst.com.
