Sensors

Is the check engine light on? What you need to do is called a "drive cycle", well maybe not, everyday driving around town and the highway will accomplish the same thing. What happens when you reset the check engine light to off is that all the monitors reset also to "not ready status". It's all due to the emission laws, what this does is prevent you from turning off the check engine light and taking the car for emissions testing, it will not pass if any monitors have not run, the monitors check the various components of the emission controls to make sure they are performing their respective functions, most vehicles have I think from 6 to 11 different ones depending on vehicle. The "drive cycle" I mentioned above is a manufacturer-specific set of circumstances that will cause the monitors to run. Some monitors run every warm-up cycle, some are continuously running. They must all show"done" in order to qualify for emissions testing success.