Check Engine Light and 02 Sensor Codes



Recently, the pesky check engine light appeared on my 2002 CLK 320. Took it to the mechanic, he scanned it and handed me a piece of paper with the following codes:

*P0135 (4) Left 02 Sensor (before cat) Open circuit in sensor heater line

*P1050 (4) Left 02 Sensor (before cat) Open circuit

He “cleared” the above codes and sent me on my way to “wait and watch.” It’s been one day and about 100 miles thus far, and no check engine light yet. I’ve noticed no change in performance or mileage. However, I’m just not comfortable with this “wait and watch” approach because I just had him put a pricey catalytic converter on one month ago.

Any ideas about what’s really going on with my ride and what I can do to prevent this from turning into a costly disaster? I understand this could be a number of things, so I would rather be proactive about this situation than reactive.


CLK Couped


What prompted your mechanic to install the pricey catalytic converter a month ago? Was the CEL on then? What was the code at the time?


There is likely something wrong with the wiring / electrical connector to the front oxygen sensor. The wiring and connectors to the oxygen sensor need careful visual inspection, and testing with an electrical multimeter (doing a wire shake, while doing so).
If the catalytic converter DID go bad, the engine problems which caused it are still there. Erratic indications from an oxygen sensor can be used for an excuse to change a catalytic converter.
Some mechanics can’t / won’t troubleshoot. They rely on symptom based guesses. They change parts hoping for a “fix”. You may need another mechanic who does troubleshoot.


An awful rattling noise emanating from beneath the car when idle is what prompted the two separate independent MB shops to suggest I change the cat. No, I did not have a check engine light on at ANY time while the cat rattled. So I suspect there were no codes?

I should also note for everyone that this pesky check engine light that is NOW on came on immediately after I left the car with the same mechanic for a simple transmission service due to a small leak. He brought the car out to me with the engine running and no CEL present, but when I made it to my destination, turned off the engine and then re-started it en route to another destination, the CEL came on immediately.

What does all of this mean? Could someone have toyed with my 02 sensor wiring to get me to come right back? Either way, what the heck do I do now? Wait and watch for the CEL and take it to another MB shop for help?


Couped and Confused


Thanks, hellokit! I suspect something is not right with the wiring / electrical connector as well. Funny enough, when I decided to have the cat changed, I explicitly asked my mechanic about the 02 sensors and wiring (based on reading I had done online). He assured me both were fine and there was no need to change them. He went on about the car was clean, everything looked well, nothing looked aged or worn. Then this?!


How many miles on this vehicle? Isn’t there a warranty on the catalytic converter?

If the rattling was coming from the cat it would indicate that the material inside the cat was loose or broken, in which case replacement is the only option. I can only imagine what this costs on a CLK 320.


This would not be connected to the O2 sensors, however, and is more indicative of some sort of physical damage. Has anything ever made contact with the underside while you were driving?

At this point I’m inclined toward “watchful waiting.” If the light comes back on they are going to have to start inspecting/testing the wiring and the connections. This may be a time-consuming, and therefore expensive, process.

Good luck.


The codes say, “open Circuit(s))”. The circuits INCLUDE the oxygen sensor. So, the O2 sensor could be defective; especially, considering the possible overheating the old catalytic converter went through. The new catalytic converter could be experiencing the same overheating.
If the catalytic converter is burning a large quantity of raw fuel, from an inefficient engine, you might see the catalytic converter glow red at night, and a heated floorboard above the catalytic converter.
An engine can be inefficient and still not set a trouble code. The oxygen sensor has to be operating optimally before it can even report a condition which would cause the check engine light to turn on. Also, lack of check engine light DOESN’T mean there are no problems. It just means that the conditions are below the 150% of maximum permissible noxious emissions.
You might be able to find a mechanic who understands the OBD2 emissions controls systems and how to troubleshoot them. Good luck in such a search.