Hi all.

BMW 523i, 1996, E39, 256S3. Fault code present is 203-O2 sensor bank 2 performance cyl 4,5,6. Live data shows both sensors are open loop until temp reaches 80C, then both go closed loop but only bank 1 switches, bank 2 stays at 4.6v, at this point engine runs smooth, after a few minutes bank 2 goes open loop and bank 1 remains closed loop and engine runs a bit lumpy. Fitted new sensor, still the same, swapped sensors over, still bank 2 faulty. Did notice that when i swapped the sensors over, the new one that was fitted in bank 2 was like new, i mean really clean like it had never been fitted, those three cylinders are certainly not doing anything.

Gas readings when fault present - O2 5-6%, HC 800, lambda 1.2-1.3, gas readings for the few minutes that both sensors are closed loop even though only 1 is switching are within limits (i wouldnt expect them to come right down low instantly as it just come off a fault).

Cant pin down a misfire when fault present, and the plugs all look the same. Have removed inlet manifold, inspected and replaced gasket and hoses, still the same. Fuel pressure is good. Inlet vac reading is good. Compressions are good.

My next plan is to swap injectors around to see if the fault shifts banks, but i cant see 3 injectors suddenly giving problems. The wiring shows the injectors having a common connection and then six connections to the ECU.

Ive probably done a few other tests and failed to mention them in this post.