V90 Closed Loop Fault

[Forrest]

When I first got my V90 Mk1 it put up the occasional P0422 error. About every 200 miles. Usually under light acceleration. I attributed this to the non-OEM cat. In December 2022 a new non-OEM cat was fitted by my local garage for the MOT. The P0422 error appeared to go away until a long trip in hot weather last September when it returned. It has occurred repeatedly since on about the same frequency as before. Some home-made cat cleaner may have caused it to subside for a bit, but not for long.

Trying to diagnose the problem rather than just throw parts at it I have tried monitoring live data from the ECU. It seems to run in an almost constant state of Closed Loop Fault, which, if I understand it correctly, is most likely caused by the fuel trimming (front) 02 sensor going out of the expected range. However, there also seems to be a body of online opinion that the rear (diagnostic) 02 sensor can lead to this condition. It is not possible to monitor the rear sensor via the ECU. I believe the P0422 error is also caused by a mismatch between the two sensors.

Now, I know the most obvious next step might be to change both sensors. They were not changed when the cat was, and may well be original. The car is now on circa 196k. And I do actually have new Bosch replacements for both sensors. However, I am also aware there are other possible causes of this problem. I’m not aware of a persistent misfire although the car has taken occasionally to idling badly and stalling when first turned on after being used in the morning and parked during the working day. However, it always then restarts immediately and runs fine afterwards.

Spark plugs and air filter are new in my ownership. I haven’t braved the quick release connectors to change the fuel filter yet though. I have mulled a possible problem with the ignition amplifiers but the rough running is very specific to when the car is first started and then doesn’t reappear.

Any other thoughts on this problem gratefully received.

[classicswede]

I have run my V90 without any cat and that did not cause problems at all.

What you need to do is monitor the live data to see what the sensors are doing. It could be a failing front sensor giving low readings