'89 300 SE - CEL
 

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.

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

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.

Code reading 5 "flashes."
 

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?

5 Flashes indicates the O2 sensor on your model 126.024

http://img100.imageshack.us/img100/9168/300se.png

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

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.

A new Bosch O2 sensor fixed the problem.
 

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

cel
 

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.