Quote:
Originally Posted by 4wheeling
I have a E420 with 128000 miles the check engine light keeps coming on and off. When on it stayed on for a while and when off stayed off for a while. Lately it is on and wont turn off. Using the push buttons I read code 4 something to do with air injection. The air pump seems to work ie. it comes on when cold and shuts off after a while. What is code 4 and how do I diagnose this problem. I have the home built code reader too so probing pins would be no problem.
Thank you.
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Make sure that "#4" is the only code in the DM.
As deanyel says is the AIR system; check that the vacuum line hasn't cracked or slipped off the main pump or the switchover valves . . it's a common occurance.