Please note, I'm not car saavy nor can I fix most things myself with cars.

2008 Volvo S40 with 251,000 miles. The catalytic converter was replaced with my old mechanic in March 2021 at 184,200 miles with an aftermarket CAT along with the spark plugs after the PO420 code came on. In January 2023, the check engine light came on at 219,800 miles so I took it to Autozone where it read as P0420. I took it to my newer mechanic who was different than the mechanic who did the first CAT replacement. This mechanic is still my current mechanic. The codes came up for him as ECM-9A0A, 990F and 2600 (I'm not sure if the check engine light was also still on when I brought it in). They said these codes were present due to intermittent fuel pressure sensor failure so I replaced the fuel pressure sensor and they cleared the codes after that and normal operation was restored after doing so. In March 2023, the PO420 code came back at 220,000 miles and it was recommended to replace the catalytic converter again but this time with a genuine factory Volvo CAT. No exhaust system leaks found. I replaced the CAT along with replacing the O2 sensors, spark plugs, air fuel ratio sensor, exhaust pipe gasket, etc. In March 2024, my wife had some misfiring and codes at the mechanic came up for misfiring and cat converter damage. The mechanic replaced all 5 ignition coils and after replacing the coils said we had no returning faults at this time for the catalytic converter so we were seemingly good. But he did mention if we drove it very long with the active misfire before we got it to him, the catalytic converter will likely fail prematurely due to raw fuel going into the cat and burning out the element. It was probably driven 15-20 minutes after the first misfire happened before we got it to him. Everything has been fine since then with no returning check engine lights, etc. until today.

I just got a check engine light code again today and had it read at Autozone. Code came back as P0420 with secondary code of 0192, gear selector module faulty signal. We've been having lots of rain/flooding this weekend and 5-10 miles prior to this check engine light coming on, I drove over 2 huge puddles of water. Coincidence? The guys at Autozone cleared the code so the check engine light is currently off but I'm sure it will come back. I I just don't know what else to do to fix this problem that keeps coming back. I'm not replacing the CAT for a 3rd time. Any help would be appreciated on your guys' thoughts. We've had many things fixed over time so if you have any questions, I can look back through my notes. Sidenote, the check engine light has come on at different times besides just these listed, I just was notating some of the times when it came on and we actually got them fixed. Thanks.

 

Thats a lot of km done 400 000km you must of replaced half the car ! is it worth still keeping ?

 

Update: With the check engine light still being off and not turned back on, I took it to my mechanic on 11/15 just to see if he could come up with anything. They completed diagnostic testing but found no fault codes stored within the system to be able to fix anything. As of today, the check engine light has still not turned back on since that first time it came on.

Any ideas why this happened in the first place and why it hasn't returned on it's own again yet? Thanks.

 

P0420 is saying the rear O2 sensor is seeing too much O2 from time to time - that could be due to a failed cat or a fuel/spark/air issue throwing off the fuel trim enough that the front sensor can't compensate. For this you'd need to wait to see if any other codes appear. It can also be due to having an after market cat that is not a perfect fit for the car. You can try installing a "cel-boss" which is a spacer that moves the rear O2 sensor a bit out of the exhaust flow to reduce the chance of a false positive fault code. As to the 0192, I would not be surprised if wet weather would contribute to that. You can always have your tech clean and reseat the contacts to the TCM computer to see if that helps. Also, it could be the PNP switch acting up. For that try doing a "shifter row" from time to time (key to unlock then go park to lo to park 20x fast to scuff up the contacts in the PNP switch)