1997 V90 P0422 Catalyst System Efficiency Below Threshold

[Forrest]

MOT is due in December. Last year the garage fitted a new non-OEM catalytic converter to replace the previous non-OEM cat. The O2 sensors were not changed.

Since I bought the car in April 2021 it has been throwing occasional P0422 errors. Previously, they were appearing every 200 miles or so. Always at low revs or when coming off the throttle. The new cat appeared to have fixed this until a long trip in September. Since then the error has been returning fairly regularly like before.

From a thread on another site I have identified this list of things to check.

1. O2 Sensors failure
2. Vacuum Leaks
3. Mass Air Flow (MAF) sensor failure
4. Coolant Temp Sensor
5. Exhaust Leaks
6. Retarded Spark Plug timing
7. Air Intake Leaks
8. Bad Catalytic Converter

I’m assuming it isn’t actually the cat unless the car is running really badly and has destroyed the new cat in ten months. It’s returning about 22 mpg at the moment which maybe slightly low but not horrendously so.

Today I removed and cleaned the MAF so will see how that goes.

Has anyone else had this error on a V/S90 or 960 and has any tips where else to look?

Thanks.

[Forrest]

Although the car seemed to run a bit smoother after cleaning the MAF it has not resolved the problem. The lambda light came on in the usual place on the way back from work tonight.

I tried a bit of live diagnostics using my Foxwell scanner but that is getting out of my depth. It only sees data for one O2 sensor, presumably the pre-cat one. Does the trace look normal? Hot engine at idle. It seems to be oscillating between 0.1V and 1V which I believe is normal but does it look a bit slow?

[classicswede]

One of my V90's without any cat fitted does not throw a code for the cat

You need to monitor the pre and post cat sensors to try and work out what is going go. You might have a problem with the rear sensor or the pre cat sensor might be reading low and need replacing

[Forrest]

Well, I think I can exclude the coolant temperature sensor. As far as I can make out the latest 960s had only one sensor which served both the ECU and the temperature gauge. The temperature gauge is working fine and the live monitoring from the ECU gives a sensible coolant temperature of around 97°C.

Picking up on Dai’s suggestion, I’m leaning towards replacing the O2 sensors. I have got spare Bosch ones for both positions. It’s just not a job I particularly relish and I would quite like to be convinced it’s the right solution before taking it on. I haven’t seen a value from the rear sensor in the OBDII data. This may be a limitation of my reader though.

[Forrest]

Before I set about changing sensors I decided to try something like Cataclean. I would have used Cataclean if ECP had offered home delivery on the workshop size bottle. However, instead I decided to mix my own!

There are lots of recipes on the internet and I opted for one which gave me about 2.5 pints of concoction. An interesting recipe mostly of organic solvents which I have poured into about half a tank of fuel. I won’t share the recipe publicly since not all the constituent parts are universally available. However I will report back on results.

There is also a much maligned video on YouTube where a well-known car enthusiast pours about a gallon of cellulose thinners into his petrol tank and claims it will clean the catalytic converter. What struck me while I was researching this is that the active ingredients of many commercial solutions are very similar to the ingredients in Tetrosyl Thinners so this might not be such a wild idea. Thinners is about £20-£30 for five litres. The cheapest I found Cataclean it was more than twice that price. I think my mix used about £20 of ingredients but some of them I have had in stock for a very long time.