My check engine light on my '89 300 SE came on two days ago. I disconnected the battery, with made it go out, but it came back on within about 15 minutes. The car starts, runs and idles perfectly. I can't detect any fault. I checked the duty cycle and it's right at 50%.

I spent many weeks tracking down a start,stall,start,stall,start,chug-chug,run fault, during which I replaced many parts, the last was the injectors, which cured the problem.

Where do I start to find out why the CEL is on, when the car is running perfectly? Any hint as to where to start would be greatly appreciated.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

You probably need to pull the code from the diagnostic connector.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.

Depends on the computer version.

California versions have a red LED & pushbutton on the diagnostic connector.

Federal versions do not have that diagnostic ability.
On federal versions fuel mixture & O2 sensor readings are the only way that the check engine lamp is triggered. IF the mixture is too rich OR lean the O2 sensor won't function & after 5 minutes of no sweep the check engine lamp will come on.

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MERCEDES Benz Master Guild Technician (6 TIMES)
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190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

It's a California version, with the red LED & pushbutton.

I turned the engine off, then turned the key back to the "run" position. The red LED was on. I had to hold the button down for a long time, but the light finally went out, and then flashed 5 times. I looked the reading up and it says, "Exhaust gas recirculation faulty." I didn't think the 89 300SE had EGR and I don't see one mounted to the exhaust manifold.

Does this make any sense? I installed a new O2 sensor in August, about 6,000 miles ago. Could this have gone bad and caused the reading I see?

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

5 Flashes indicates the O2 sensor on your model 126.024



I would make sure its all hooked up ok before removing it. Make sure the connector is getting voltage.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
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2003 Gray Mercedes ML350 - 122k

Aha! The code fault table I used was the one Authur Dalton had posted, and I wondered if perhaps it was for different models, but couldn't find anything else. This is a big help.

A failed O2 sensor makes sense. I had just put nearly $2,000 into the car, and got "penny wise and pound foolish" on the O2 sensor I put in. I found it on ebay for a low price and got what I paid for. I now have a new original on the way.

Thanks for pointing me in the right direction.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

I replaced the no-name universal O2 sensor with a Bosch that has the correct electrical fitting to plug into the car's wiring harness and it solved my problem. I cleared the code and the CEL light has stayed off for over 1,000 miles. Better still, I made two 600 mile trips since then and the milage is up a bit, 23.5 mpg vrs. a former best reading of 22.3 mpg. Plus I made the exact same trip the week before, with the defective O2 sensor in place and got 19.4.

The no-name universal came from an eBay seller in a box that was not marked as to the country of origin, but I suspect it was China. It's a good supplier & brand to avoid - carpartswholesale

carpa

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

I think this is whats wrong with my 90 300se how did you clear the code/light?

Hey, guys I posted a sticky (well, MB Doc posted it for me) at the top of the Tech Help thats gives the CEL error codes for Mercedes models 1988-2000. Its a pdf with pretty much every code regarding every mechanism that can trigger a fault code, please check it out.

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1990 190E 3.0L