GSJoe

I think I have a new one for the books so to speak, but I hope not. I'm wishing someone else has had or knows about the crazy CEL scenario in a 1999 300ES that has plagued me for several months now. I have an intermittent CEL!!!! Yes folks, the car runs fine, but at random the CEL will come on. It may go off after a day or so and may stay off for two weeks or more. Suddenly, however, the light comes back on. There seems to be neither rhyme nor reason as to when it will come on, how long it will remain on, when it goes off or how long it will remain off. Mind you, all of this is happening without anyone having canceled fault codes at any time. The dealer first thought it was the gas cap, but after several replacements the intermittent problem remains. I am being told that other than a bad gas cap, the ECM will not allow the CEL to go out without the fault first being fixed and then having the fault code or codes erased. Any ideas, anyone??

PuReChaos

It would be helpful to know the CEL code that is appearing.. also CEL can go off without being erased, it the problem does not occur for whatever reason after a few trips the CEL will turn off. I believe its 40 cycles of the key on without the fault being detected. intermittent problems are incredibly annoying, especially if you don't have any noticeable driving irregularities. If I were you I would buy a cheap DTC reader online and next time the light comes on check what the fault code is. You could also probably ask the dealership as when you bring the car in i assume you bring it with the light on. trouble codes also get stored in memory for a little while and would be super simple for them to bring up. if you know the trouble code please post it so we can assist you further!

GSJoe

Thank you for reading and responding to my post. At the moment the light is off, but when it comes on again I'll have the system scanned and will post whatever codes are found.

Hayk

iTrader: (1)

Even when the light is off, the code gets stored in the system. If you never erased it, you will be able to retrieve it.

GSJoe

I managed to find time to get the car to a parts store today and have the ecm scanned. The only code showing is a 0420. Could this make for an intermittent CEL? I'm confused. The owner of an independent repair shop I often use suggested I probably only needed an O2 sensor when I called him today with that code information. Could that possibly be the case? It's the intermittent part that confuses me. I thought the code would have to be cleared before the CEL would turn off. In my car, however, the light turns on and off at random. I'd be happy to spend the $120 he quoted me for part and labor if this is a reasonable diagnosis.

jltait

Sounds like a good diagnosis. A new O2 sensor is probably your fix. It's not uncommon for these to go after a few years and an intermittent CEL with a failing sensor is not uncommon either. Good luck and Godspeed.

Hayk

iTrader: (1)

P0420 is a catalytic converter code. Good luck.