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Your check engine light will not come on in the auxilary (accesory) position. It comes on when the key is in the on (run) position to check the warning lights just before you start the vehicle.
Your bad muffler might be part of the reason your check engine light comes on. Usually, your bad muffler causes something else to go wrong and that causes your check engine light to come on. Problems cascade.
Your mom.
There are pages of causes, you need to have it checked for codes.
See related question below.
gage of temperature go to hot position but is not hot the engine check gages light come on
Lots of problems could cause the light to come on. First off, check your fluid levels; second, take it to a mechanic.
The Check Engine light comes on when the ECU detects a problem with the emissions system. Seek professional help.
Have the code read, that will tell you.
Low fluid levels in engine.
A problem with the emissions system.
you have a problem with your engine and and its emissions. take it to auto zone for a free check.