C180k with P0137 & P0172

[Lyserman]

Quick recap:
EML came on, Downstream O2 sensor fault, replaced.
Failed MOT on emmissions, lambda low, running rich & stinking of ammonia from the exhaust.
I have renewed the check valve and perished pipework under the inlet manifold, very cramped behind there but doable without removal of the compressor. I could do a mini write-up if anyone is interested.
Between ordering the pipework parts and their arrival I temporarily jumpered out the broken check valve and perished pipework and took the car to a local indie for a diagnostic. Said what I had done so far and afterwards he said it was P0172 (self adaptation is too rich and P0137 downstream O2 sensor short circuit to ground).
He said either wiring fault or ECU.
I used the plug from the old sensor and checked all four wires for continuity back up to the ECU plug, all 4 had good continuity but the thicker red wire leading from the sensor sockt end has a resistence to ground of 431Kohm whilst the remaining three are in the 45Mohm range.
Before I start pulling the loom to check for faults does anyone here know if the 4 wires to the sensor just go straight back to the ECU plug or is there somewhere else it travels to enroute?

Sorry for the long-winded post but had to put in the relevant details.

Thanks,
Tony

 

[WG M-B]

Only times I have had this issue it seems to have been caused by oil in the wiring harness at the downstream plug. Is the connector free from oil residue?
Once the work has been done an ecu re flash is usually needed

 

[Lyserman]

Thanks for the reply.
There was very little oil residue in the plug when I originally changed the downstream sensor. A smear and not sodden. I cleaned it out and none has returned in the few days since then. I would have thought that more would have appeared by now if it was contaminated.
There is no evidence of oil contamination in anywhere else in the loom even at the camshaft magnets and nothing in the ECU plugs.

 

[Lyserman]

Okay!
We have finally got to the bottom of this particular M271 problem. Here is another solution to add to the chain of fault-finding for this nasty little fault.
I took it to a local indie and after many hours tracing wiring, changing components in the chain and even reflashing the ECU after I passed on the recommendation from Wayne.
He was just about to declare a faulty ECU when he thought to check the connections in the plug to the ECU. He found that the crimp connections were loose. Re-crimped them and instantly the car ran perfectly.
Further good news is that when they went to change the timing chain he found it was as-new, so it must have been changed before my ownership and not shown in the paperwork.
Hope this info helps someone else out with similar problems.
Tony

 

[AMGeed]

Glad to hear you finally found the solution to your problem.

 

[Lyserman]

Glad to hear you finally found the solution to your problem.

You are not the only one