What to do when you see a check engine light in a Ford Expedition XLT?

It depends the check engine light comes on usually when there is an issues with the computer that is related to misfiring, O2 sensor, etc. You can buy at an Auto parts store a code reader for $75 or you can pay yoru mechanic $75 to check the check engine light. I bought the code reader since once my check engine light came on it keep coming on for different reasons, but mainly for failure of coils and O2 sensors. If you live in a state where they require yearly emission checks then you can take it in and it will produce a failure which tells the fault. In my state that is $23.50, but you fix the issue and take back and they give you the inscpection sticker for no extra charge.. But the only way to find out why the light is one is to runt he computer and check the code.. good luck You can also get your codes read for free at Advanced Auto or Autozone. ANSWER I had this happen for quite some time, always had a diagnostic test come back clean. I was confused for some time, but found out that it was coming on because the gas cap wasn't tightened enough. Check to make sure it's on tight, or your check engine light will stay on if it isn't. This was an easy fix. I always check the minor things before thinking it's something major, because most of time it has been little things.