Here are the possibilities and how to approach them:
* Check Engine Light (CEL) is on: This is the most common indicator of an emissions system problem. You *must* address the CEL before attempting any other fixes. Get the diagnostic trouble codes (DTCs) read by a scan tool (available at auto parts stores or mechanics). The codes pinpoint the specific problem, be it a faulty sensor (oxygen sensor, mass airflow sensor, etc.), a vacuum leak, or a problem with the catalytic converter. Fixing the underlying issue is the "reset."
* Specific Emission-Related Problem: Describe the specific issue you are having. For example: Are you experiencing rough idling, poor fuel economy, or failure to pass an emissions test? The solution depends entirely on the symptom.
* Battery Disconnect: Disconnecting the battery's negative terminal for 15-30 minutes can sometimes reset the PCM. This clears short-term memory but won't fix a faulty sensor or component. This is a last resort and *only* after you've attempted to diagnose the actual problem.
* Incorrect terminology: It's possible you're using the wrong term. Double-check your repair manual or consult a Jeep YJ forum to ensure you're identifying the correct component. There's no singular "emission timer relay" like in some other vehicles.
In short: There's no single reset button. Diagnosing and fixing the *cause* of the emission system problem is the only true "reset." Get the DTCs read, and that will guide you (or a mechanic) toward a solution. Attempting to "reset" without knowing the cause is a waste of time and could potentially damage your vehicle.
