Mysterious check engine light
 

I have a friend who has a 1995 C280 with a check engine light that never goes off. He takes his car to the shop and they run a diagnostic test but they find nothing wrong with the car. So the shop resets the computer and sends him on his way. A few days later the light is back on. Any idea what could be casuing this?

If there is a CE lamp, then there is a stored fault code.. that is what a CE is .. a detected fault indicator ..
Have him ask what codes they find when they scan the Diagnostic Module...

He said they found no codes but the light was still on.

Time to use a different or newer computer to check the codes! Try the dealer

That is not what you said in the first post..you said they reset and the light went out and came on later.
If it came on later , there is a code to go along with the light.
The CE on that model has to be gotten from the 16 pin connector under the dash, not the 38 pin under the hood.
If they have no codes from the 38 pin , that has nothing to do with the codes stored for the CE ...

That sounds like the problem - they didn't know it was an early OBDII.

Also in that model, a faulty OVP relay can cause flakey codes, i.e. differenct codes after resetting CEL.

Problem fixed
 

Thanks guys.. the dealership found out what was wrong and fixed... my friend decided to get a new mechanic

Well,
Thanks for the through feedback.