Originally posted by r0ckstarr
That only works on OBD1 systems. They changed it in OBDII.
No they didn't. What happens or is supposed to happen is a trip service code is set when the car/processor is first powered up (removing and reinstalling the battery cable or resetting the processor). In order to pass an inspection in states that do them that code (P1000) has to be cleared. To clear that code one must complete 7 or 8 tests (depending on the type and model year of the car beginning in 1996 for all vehicles). Tests that determine the working condition of all of the electrical components as well as tests that determine the condition and efficiency of the catalytic converter. A Check Engine light turning on indicates a failure, Multiple times, of a component that will drive vehicle emissions above a set standard (that standard is different in some states, and is different in other countries, and is different for different sorts of vehicles, car vs truck, gas vs diesel, etc). There are, or Were 2 different sets of standards in the United States. One is set by CARB (California Air Resource Board) and the other, sometimes somewhat looser, is set by the EPA.
A failure code turning the check engine light on requires 2 trips (a drive cycle that takes OBDII through a complete check of all of the monitors). An OBDII trip can be completed in a single drive cycle or it can be completed in multiple drive cycles (turning the car on and driving it somewhere). If a failure occurs on the first trip, a pre-code will be stored. If the failure occurs on the second trip, it turns the check engine light on. The light will stay on until one of 2 things happens. It is cleared by a diagnostic system (or removing the battery cable), or the problem cures itself (it happens). For the latter, 3 consecutive trips without a failure are required. In This case, the code will be stored in background memory for 40 trips.
The main exception to the multiple trip rule is (or was) an engine Misfire (usually caused by a bad spark circuit) that the software deems dangerous to the converter. A cylinder misfiring will dump raw fuel or very fuel rich gas down the exhaust pipe causing the converter to abnormally heat up. It will heat to a point where the catalytic converter brick (ceramic with a precious metal coating) will disintegrate. To protect the converter in this situation, the processor will turn off the fuel and spark to the offending cylinder. The indication will be a Flashing Check engine light while it is happening, followed by the light just staying on indicating that a problem has occurred. A misfire code can also be set by a malfunctioning injector. There are two types of codes for this monitor, one is considered catastrophic to the converter, the other is not. The former will flash the check engine light.
Codes from regular engine electrical components (Engine Coolant Temp (ECT) and Air Charge Temp sensors for example) will set and the software will estimate what Should be for the failed part. In other words, if the engine coolant temperature fails, it will use the reading from the air charge temp sensor to best guess what the ECT should read. If on a V type engine, the Oxygen sensor should fail on one side, the software will use the readings from the other side to help control fuel/air mixture (one of the purposes of the oxygen sensors) on the failed side to remain in closed loop fuel control.
I was an OBDII Calibration engineer for over 10 years.