I have owned my 1990 300 SE for a little over a year. The car runs beautifully, has 125,000 miles, and the only problem I have had is a code and light on for the O2 Sensor. I put a new Bosch sensor in shortly after getting the car, cleared the code, and it ran perfectly for about 9 months and no light on. Then the CE light came on and the code for O2 Sensor grounded was on. I replaced the sensor again, cleared code, CE light off, great! But the light has come on again, code for O2 is read. I have a pretty good oil leak from rear main seal that gets on the exhaust and will cause some smoke at times off the exhaust pipe. Could this burning oil affect the O2 sensor? Also, what happens if I run the car with the sensor not working? Thanks for any advice!

OK, I give up, it seems as if just about any question can be answered on this forum, except this simple one. Does anyone even know what an O2 Sensor does? I would like to know. How about what tires are rounder?

The O2 sensor reads the oxygen levels in the exhaust to tell the engine computer how rich the engine is running, it can then adjust the fuel mixture to run optimally (and as clean as possible) Oil spilling on the outside of the exhaust would not affect it. If the engine itself is burning oil then it would potentially ruin the O2 sensor quickly.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Bigw, sorry we ignored you.
I've never faced that fail code, but I'll give my thoughts.

Well, let's see, I suppose a grounded O2 sensor could be caused by:
- A faulty KE control unit - seems unlikely since it ran fine for so long.
- A damaged wire - easy to check, at least up to the connector under the floorboard. When you replaced the sensors, did you buy a plug-in unit, or splice in a generic unit? Either way, tracing the wires from the sensor up thru the connector as far as you can would be a pretty easy check. If you spliced in the sensors, you might check the connection, be sure to solder them
- A badly contaminated sensor might cause the CEL. I'm not sure it would, but I guess if it got coated enough, perhaps it could.
How did the old units look? Were they pretty coated?
The M103 engine is notorious for burning oil thru the valve stem seals. How is your oil consumption? If the sensor were exposed to enough gunk long enough, it might set the CEL.

One thing you might do is buy a little Sears multimeter that has the "duty cycle" function, and monitor the diagnostic computer terminals for injection system duty cycle. This will give you an idea of the system efficiency, and how the sensor is functioning. Do a search here for diagnosing the injection systems - there are some good write-ups that really help to understand
how it works.

You could also actually monitor the voltage output of the sensor while the engine's running, by tying a sensitive voltmeter into the sensor pigtail at the connector. Some threads herein on that technique also.

Basically, if the O2 sensor is not functioning, the system will revert back to a preset mixture setting, which is reasonably close, but certainly not as efficient as the O2 monitored system. GM refers to it as the "limp home mode". You will see less economy, less smooth performance; kinda like an old carburated engine.

DG

DG

Thanks for the replies, it is a Bosch Sensor, factory plugin. I do not think that the engine burns any oil, just leaks from rear seal. The sensors that I replaced looked pretty clean at the probe. Thanks again for the replies.

One more thought; when you installed the new sensors, how much did you torque them? If you really leaned on them, perhaps that contributed to the shorting failures of both? Unlikely, but we look for the simple things first.
Don't remember the torque, but it's 30 lb-ft for Toyotas.

Oh, in all my years of messing with cars, I finally saw my first CEL due to a faulty O2 sensor, just this weekend. I know it's common, but I guess I've been lucky. My wife's sister has an '01 Camry, and it threw the code Saturday. Sure enough, O2 grounds shorted. It cost $180 at the local parts store!!!

Go ahead and fix that rear main leak, it only costs $20 (and pulling the tranny)! Oh, are you sure it's the rear main? The 103 is infamous for leaking valve cover gaskets that run down and drip of the rear main area - fooled me the first time.

DG