P0137 is back.

[Lyserman]

Well after thinking the O2 sensor issues were sorted, the first time I drove it from cold the EML came back on, P0137.
I have changed all the usual things, vac hose and check valve under the inlet manifold. ECU re-flashed. Even renewed the pipe and check valve that feeds the evaporative system as that check valve had stopped functioning. No sign of oil contamination in the loom.
So today I am removing the starter motor and other ancilliary bits to check the wiring loom from the downstream sensor back up.
Any further suggestions from the audience would be most appreciated?

Thanks,

Tony

 

[Lyserman]

Well after much wire chasing and having a chat with Olly at PCS (Many thanks for taking the time to talk this through) we came to the conclusion the next thing was ECU replacement .
I pulled off the ECU and sent it to my brother-in-law who is an electronics wizz and within 5 minutes of him getting it he phoned to say he thinks he has found the issue.
Pin 7 in the large connector socket was covered in white corrosion products (why didn't I spot it? ) and that is the one that receives the mV signal from the downstream O2 sensor.
I have just tried the returned ECU in the car and I am now getting 700mV from the sensor whilst idling :bannana: prior to the clean up it was struggling to achieve 300mV when it was showing anything.
So there is another thing to check if your M271 throws up a P0137.

Tony

 

[Lyserman]

I am at my wits end with this fault.
Sorry for the long post but I want to get all the relevent details in.

I have STAR so I can see what the O2 sensors are doing, which is:
Starting from cold.
Upstream sensor shows heating capacity "controlled" and 1200mV, within spec.
Downstream sensor shows heating capacity "reduced" and 443 mV but after a couple of minutes the heating capacity changes to "rising" the sensor output then falls to 0mV. I had a voltmeter connected to the sensor, it showed -220mV but STAR showed 0mV.
That is when the P0137 appears. Once the car has warmed up the sensor sometimes starts putting out 700mV and all seems well. but the management light comes on once I start driving the next time.
There was no evidence of oil contamination in the loom or ECU socket but there was a tiny amount in the socket for the downstream O2 sensor. This was cleaned out when the sensor was changed and has not returned.
So far I have renewed:
Downstream O2 sensor with genuine MB.
The vacuum pipes and check valve behind the supercharger (old ones were perished).
Pipe and check valve to petrol tank carbon canister (check valve was not checking).
Breather pipe from camshaft cover (old one was soft but in one piece).
My findings:
Pin 7 on the ECU was discoloured with white corrosion (this is the O2 sensor input to the ECU) so this was cleaned off.
Several of the female connectors in the ECU plug were slack so these have been renewed (£1.90 each

).
I have run temporary wires from the sensor heater to the ECU to see if the wiring was causing the issue but it was still the same.
If anyone has anything further they could add I would be most appreciative as the next step is to renew the ECU, which although is very easy to physically change, it needs a Mercedes connected garage to swap the info from the old unit.
If anyone knows of somewhere that can bench test the ECU (apart from ACtronics) I will get it tested.
Many thanks for looking.

Tony

 

[Lyserman]

Well would you Adam and Eve it?
In desperation I replaced the new MB O2 sensor with a Bosch sensor (they are identical except for the MB part number engraved on the OEM) and the problem has not re-occured in two days, including a 300 mile journey north and several short journeys today.
I got my Brother-in-Law to test the MB sensor on his oscilloscope with a 12V power source for the heater, we discovered it is sending out a -ve mV signal every time we test it.
The sensor is going back to MB on Monday as it is only a week or so old.
I am so relieved to finally find a genuine fault instead of chasing phantoms and have learnt not to accept that just because it is new it must be working.
If the engine light remains off until I get home on Monday then I will put this to bed and mark it solved.

Tony

 

[Twistedmind]

Good news. Well done !!!

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