94 E420 - CEL on, DM will not display codes - PeachParts Mercedes-Benz Forum

willjohn123 · #1 02-05-2008, 10:21 AM

94 E420 120k. A couple of weeks ago I left the intake air temp sensor unplugged by mistake. CEL came on after a few trips. I plugged the sensor back in and tried to retrieve the codes using both the built in reader and the home built one. The DM would not respond at all. It smells like it is running rich and I am getting horrible MPGs. I hoped that it was the O2 sensor so I replaced that this weekend and it did not help. Without being able to read the codes I need to go about this differently so here is my question:

What can go bad and cause the CEL to come on, and bad gas mileage, but without causing the car to start or run poorly?

JimF · #2 02-05-2008, 11:08 AM

The MAF can if it's dirty/bad/contaminated. Checkout MENU#4a for details.

willjohn123 · #3 02-05-2008, 11:52 AM

Thanks Jim, that is very helpful. I wish I would have read your page before I spent the $ on a new o2 sensor.
If I wanted to test the o2 while in the car, will pin 14 in the 38 pin connector display the .1v to .8v or would I have to tap into the o2 sensor wire itself?

JimF · #4 02-06-2008, 11:44 AM

With 120K miles on the car, you didn't waste money since the it was "time" to replace it.

Since you have a "LH" system, the MAF may well clean up using the MAF cleaner described in MENU#4a. It's worth a try. The "wire" versions clean up better than the "hot-film" MAFs.

I don't think you will see a 'pin' in socket #14 but you should find the "X11" connector on the driver's fender-well.

willjohn123 · #5 02-06-2008, 04:42 PM

My o2 sensor was replaced at 96k. Working spare parts are always a good thing to have though. At least it was quick and easy to change. I picked up a can of the CRC MAF cleaner yesterday. I will clean the MAFs on all of my fuel injected cars this weekend and see what happens. As for the pins, I was refering to pin 14 in the X11 connector which faintly lights the home built sw/led when stuck in that pin. I will check the voltage as soon as I borrow a digital meter. Thanks again

JimF · #6 02-06-2008, 07:58 PM

Quote:

Originally Posted by willjohn123

As for the pins, I was refering to pin 14 in the X11 connector which faintly lights the home built sw/led when stuck in that pin. I will check the voltage as soon as I borrow a digital meter. Thanks again

I know what you meant but you have the connectors confused; The 38pin Diag Connector is "X11/4". . . the "Lambda" connector is "X11" and it s/b on the driver's side wheel well. It has a special connector and the O2 signal is on pin "3" with pin "2" as ground.

Supposedly, in "4/94", MB discontinued the "X11" connector and moved the O2 Lambda signal to pin 14 on X11/4. If you see a pin in that position, then you're all set.

willjohn123 · #7 02-11-2008, 12:09 PM

Cleaned the MAF this weekend. It did not seem to help. Is there a simple way to test if these are bad without a scanner? The link to testing instructions on Jim's page does not work. Can I learn anything by unplugging the MAF connector and starting the engine?