My friend has a 99 SLK with 28000 miles and the "check engine" light keeps coming on. It has been in and out of the shop a couple of times and it keeps coming on. I am sure (at least I assume) the mechanics are checking the vital signs each time and tha car runs fine.

Techies, what does this mean? What are the causes for the check engine light? Could it be something as simple as a bad reset module that is not keeping its reset info?

Any ideas would be appreciated

Don

Had this happen recently on '98 E320. Turned put to be Mass Air Flow Sensor. Price - $300 + installation. Yours, however, should still be in warranty.

The lambda (or O2) sensor can also cause the CE light to come on...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

The check engine light lets the operator know that a fault has occured in the Emissions system and it has been detected by the diagnostic module.
So, the ONLY way to start diagnoses is to get the codes from this module. That is all the ck eng lite is trying to telling you .
There can be upwards of 60 codes and then they have to be
interpreted. [ the tricky part]
Do not change a part on your car cuz someones else tells you there light went away when they changed part "XYZ".
You will run out of Money.....

I have a 98 SLK and had it in four times to get the check engine light repaired. It finally turned out to be the electronic control unit itself that was giving a false indication. Since the electronics have been replaced (about a year ago), it has been fine.