Hi guys, 

I have a problem that I have had for a while and im starting to get a bit desperate with it really. I have fault codes P0136 and P0141, related to the cars o2 circuits. I have replaced the original downsteam o2 sensor with a new Bosh one, but the problem still persists. I took the car to BMW and they did a diagnosis, stating that it needs a new sensor, but obviously I have already put a new one in. Is it possible that only BMW will have some kind of sensor that works with my car and that an after market one like the one i bought (new from eurocar parts) wont work?

They stated that if they change the sensor and this does not fix the problem the car may need a new DMU, which will costs £1000 plus, and i refuse to believe my car needs this as it runs perfectly and achieves great mpg. 

Any help would be really appreciated. 

Morning Michael

Your first code refers to the O2 sensor the second  just confirms that the ECU isn't seeing the expected values from the sensor.

Sadly many garages no longer fix/repair anything they read codes and change parts, faster for them (not to say more profitable) 

So back to your codes PO136 remember this tells you why the light is on as it isn't seeing the correct values it does NOT mean the sensor is broken. Things that can trigger the sensor are (1) Frayed wiring in the loom leading to it (2) Exhaust leaks ahead of the sensor (3) Inlet manifold leaks or vacuum leaks on the inlet side (4) Rusty leaking boss that the sensor mounts to. PO141 Confirms that the values are out of sync and should clear once the problem is repaired.

I would begin at the sensor, is it correct for your car? Did you check www.realoem.com to make sure you have the correct sensor or is it a "generic" one assuming it is correct then start testing for leaks, inlet and exhaust, test the wiring carefully looking for any sign of chafing or fraying, with a multi meter test for continuity. Online you should be able to find the test procedure for the sensor and the correct values.

Good luck

Dave