Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
do you have the fault code that was generated?
does the car run / drive OK when the check engine light is out?
OK as you've already replaced coils and plugs then I'd suggest the next step is to measure fuel pressure on a running engine if its less than
If thats OK then check over the exhaust for leaks before the Oxygen sensor as
this has beenexperiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.
this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst
The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers
Ok thanks for the extra info, I' agree that a clogged / broken cat is a possibility
If you remove the oxygen sensor in front of the cat you can possibly peer inside to view the condition of the cat honeycomb, if its anything other than uniform and flat then it will need replacing