* Resetting the Check Engine Light (CEL): You can't simply reset the CEL. The light indicates a problem detected by the onboard diagnostic system (OBD-II). You need to *fix the problem* causing the CEL. Once the problem is fixed and the car has run for a while, the light will often turn off automatically. A code reader can tell you what the problem is. Simply disconnecting the battery won't solve the underlying issue; it will just temporarily clear the code.
* Resetting the Trip Odometer: This is usually done using buttons on the instrument cluster. Look for a small button on your odometer, possibly labeled "TRIP" or with a small triangle or similar symbol. Pressing it repeatedly should cycle through different trip odometer readings or reset them. Your owner's manual will have the exact instructions.
* Resetting the Average Fuel Economy: Similar to resetting the trip odometer, there's usually a button sequence or method to reset the average fuel economy display. Consult your owner's manual.
* Clearing other computer memory (rarely needed): Disconnecting the negative (-) terminal of the car battery for about 30 minutes will reset some computer memory. This is a last resort, and shouldn't be done unless you've tried other methods and have a specific reason. Caution: This will also reset your radio presets and possibly other personalized settings.
In short: Tell me *what* you want to reset on your 1999 Chrysler 300M (CEL, trip odometer, average fuel economy, etc.), and I can give you more specific instructions. Your owner's manual is the best resource for this type of information.
