2001 VW Jetta

My son who lives on Maryland’s Eastern Shore has a 2001 VW Jetta

which the last couple of times he has come to W. Maryland to visit has had the “check engine” light come on coming up the last mountain. It goes off on the downside. Our regular mechanic has checked it both times and says nothing is wrong. Any ideas?

Are there any symptoms other than the light? Does the engine overheat climbing the mountain? Are any of the fluids low?

If the light was on there should be codes in the computer. Did the mechanic tell you what the codes were?