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The "Service" light is the only one that comes on at regular intervals (as an oil change reminder).
The "Check Engine" light indicates that one of the ECU components has an operational problem or has detected an unusual engine running condition. When "Check Engine" is lit there should be a diagnostic code (or multiple codes) stored either in the main fuel injection controller (ECU/FCU), the ignition controller (ICU) or the ABS controller (if present). At the diagnostic socket you need to separately interrogate each controller with the jumper wire in socket #2, #6 and #3 respectively. If you've read the 700/900 FAQ here you'll already know how to do that and that you keep reading codes until you get a 1-1-1 null code or the first code is seen again. You can read through the codes as many times as you want. Also note that not all codes will cause the "Check Engine" light to go on so it's a good idea to check for codes at each service interval or whenever you want to impress somone with your skills.
You reset the "Check Engine" light and clear the codes (whether or not you found any) by following the procedure (in the FAQ), again separately for each controller. You can also do a reset by disconnecting the battery and waiting a few moments to let the controllers discharge, but you may then need to re-enter the radio security code. Discharging the ECU has the added advantage of causing recent sensor readings stored in memory to be lost forcing the engine to start by retraining itself which can sometimes temporarily make sluggish operation go away (an indication that a service/tune up is required). If you do find a code that doesn't make particular sense and the engine is otherwise operating just fine then it is often a good idea to simply reset the controllers and see if the code reappears. If a code doesn't reappear then it may have been set for a transient problem or under abnormal operating conditions (like when someone disconnects the AMM while the engine is running) and you can forget about it.
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Dave -not to be confused with a real expert, just goofing around at this
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