What year, engine and 4WD system do you have?
The Check Engine Light (CEL) means there is a problem that was detected and needs to be corrected.
Creek already talked about the Engine Oil Change Required Message that will pop up in the EVIC on the Dash, it does NOT lite the CEL. Earlier Commanders, I believe it just counts down the miles between oil changes, later Commanders the PCM actually runs an algorithm to judge oil life based on the engine usage and conditions. There is no percentage indicator, you simply get the warning pop when its time to change the oil. You can reset it yourself easily if you change your own oil.
Already mentioned is the Key Dance to get the PCM to spit out stored fault codes, and likely the fault codes are the source of the CEL. The key dance does NOT spit out every code and especially very few or none of the codes for the other electronic systems. The manufacturers keep that proprietary to force you into the dealerships to pay for overpriced diagnostics and service, instead of making it easy for you to read the codes yourself.
If you're overdue for new plugs, you might be getting misfire codes, the PCM will detect before you can. Fresh plugs will regain some lost power and fuel economy and smoother idle and engine running as well, if the exiting plugs are burned away.
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