P0137 low voltage o2 sensor bank 1 sensor 2 W205

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OldskoolRS

Active Member
Joined
Jun 22, 2022
Messages
187
Location
Berkshire
Car
C200
EML came on last month on our W205 C200 petrol 7G auto. I have an iCarsoft reader and it gave the error low voltage o2 sensor bank 1 sensor 2 which I cleared and it came back after a couple of drives from cold.

I've replaced the o2 sensor (Bosch part) and cleared the error. As the service was due I did an oil and filter change and also replaced the spark plugs as the car is at 38,000 miles and now 8 years old (2016). I thought this had cracked it, but after 5 drives (from completely cold) it came on again today. MOT is due next week and while I think I could get through by resetting it immediately before the test, obviously I want to get to the bottom of it.

I gather 'Star' readers might show more details, so I may have to take it to a local Mercedes specialist (I won't use the main dealer having bad experience of them from when I had company cars before I retired). I'm happy to do any further physical/visual checks before I get to that stage, hence posting on here to see if any suggestions or others who have come across this?
 
O2 sensor 2 is the sensor after the catalytic converter. This suggests that the mixture is lean - too much oxygen, maybe an air leak into the exhaust system somewhere, maybe the ECU has reached the rich limit for closed loop control (but for some reason insufficient petrol is being injected still - low fuel pressure, knackered injectors?). Check the actual O2 sensor 2 reading - there may be a break in the witing to it. Check the readings for O2 sensor 1 (pre-cat). If this is switching normally (0.2 - 0.8 volts every few seconds), with a lambda reading of roughly 1.00 (once the engine has warmed up).

The post-cat O2 sensor should normally give a steady reading of around 0.5 volts or so.

If a physical check of the exhaust manifold/pipes/cat looks OK, and no visible leaks on the inlet manifold, and the fuel pressure is OK, then it might be time to take it a specialist.

Oxygen sensor - Wikipedia . I'm sure that there are better explanations available on the web.

Ian.
 
Check the actual O2 sensor 2 reading - there may be a break in the witing to it. Check the readings for O2 sensor 1 (pre-cat). If this is switching normally (0.2 - 0.8 volts every few seconds), with a lambda reading of roughly 1.00 (once the engine has warmed up).

The post-cat O2 sensor should normally give a steady reading of around 0.5 volts or so.
Thanks for the reply IanW.

I've checked the wiring from the sensor as best I can (it disappears into the loom in the depths of the engine bay). Also checked for good earth continuity to the body of the sensor and that was good at near enough 0 ohm. The Pre-cat sensor switches as you say once warmed up as both pre and post cat sensors seem to start on a fixed value while the system is in OL mode during warm up (1.275v pre cat and 0.200v post cat seem to be the typical 'from cold' readings).

Very occasionally when starting from cold the post cat o2 sensor reads 0.000v during OL mode, but not sure if this is a red herring.

I've driven the car while my wife monitors the live readings and the two sensors seem to move up and down in sync once warmed up, the pre cat value typically higher and the lambda reading around 1.000 too.

I've had a good look at the exhaust from underneath and can't see or hear anything that indicates a leak. The only thing I have noticed is a very high pitched squeal when the engine is turned off, but it's been doing this for a long time before the EML started coming on. The squeal stops when the throttle body does some kind of reset (I can hear a clicking from that area after turning off), but sometimes comes back again. I've tried to locate the squeal but as it stops randomly it's hard to tell if pushing on a pipe/part is affecting it.

I suspect I may have to give in and take it to a specialist with Star. I just hate giving in, plus I don't want them to start throwing parts at the car in the hope of fixing it, so I want to be confident I've checked everything I can first. At this point I'll have to put it through the MOT and see if anything is picked up. We've got a family visit coming up, so I won't have long after the MOT to get any repair work done as I timed it badly. Don't fancy a 300 mile drive in our Alfa Mito, so might end up hiring a car at this rate. :(

The EML comes on sooner when my wife drive it...I tend to drive it more gently, whereas she will accelerate much harder between traffic lights. The EML can come on within 2 drives with her and can take 5+ with me, but I don't know what to make of that. :D
 
If the catalyst is working correctly you shouldn’t see the pre and post sensors following each other. The pre catalyst sensor should roughly follow a sine wave. The post catalyst sensor should be reading a low figure if the catalyst is doing it’s job
 
If the catalyst is working correctly you shouldn’t see the pre and post sensors following each other. The pre catalyst sensor should roughly follow a sine wave. The post catalyst sensor should be reading a low figure if the catalyst is doing it’s job
Doesn't sound good then. Very disappointing if the cat has only lasted 38,000 miles. :(

Sometimes the rear o2 sensor sits around 0.200v with only slight variations while the pre cat one moves up and down, but it's inconsistent. I have run some cat cleaner through the car, but didn't seem to make any difference (always think additives are a bit clutching at straws to be honest).

I guess it may fail the MOT on emissions if the cat isn't working properly, regardless of the EML being on or not at the time.

On the plus side I can replace it myself, on the bad side it won't be cheap if it turns out to be the problem. :(
 
Doesn't sound good then. Very disappointing if the cat has only lasted 38,000 miles. :(

Sometimes the rear o2 sensor sits around 0.200v with only slight variations while the pre cat one moves up and down, but it's inconsistent. I have run some cat cleaner through the car, but didn't seem to make any difference (always think additives are a bit clutching at straws to be honest).

I guess it may fail the MOT on emissions if the cat isn't working properly, regardless of the EML being on or not at the time.

On the plus side I can replace it myself, on the bad side it won't be cheap if it turns out to be the problem. :(
Of course an illuminated EML will be a fail regardless.
 
Of course an illuminated EML will be a fail regardless.
Yes I appreciate that, though typically I can clear the EML and it'll stay off for a few days, so I'm just hoping I can get through the MOT with the light off. Then I can work to resolve the issue in my own time.
 

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