P0420 code troubleshooting
 

Hey,
Recently had a P0420 code come up on my 2004 V70 2.5T. Otherwise known in Volvoland as ECM-4801 - Three-way catalytic converter efficiency, Bank 1.

-The Cat is only ~1 year old.
-I don't hear exhaust leaks
-fuel trim looks normal

I took a readout of my oxygen sensors (while at idle and revving the engine momentarily) which you can see in the attached video.
The front wideband sensor looks like it is doing okay, trying to stay around 1.00. However the rear sensor looks to be jumping around quite a bit. Starting at idle ~0.1 then jumps around rapidly to max of ~0.8 then keeps moving around.

Do those rear values look normal to anyone or is my code likely due to that rear sensor?
Thanks

Did you use after market parts when replacing the cat? Did you replace the O2 sensors when the cat was replaced -again were they OEM or after market? These cars are very sensitive - as noted an intake air leak or exhaust leak can fool the sensors, plus whatever contaminated the cat in the first place (like a leaky turbo seal?) may have contaminated the new cat or the sensors. If the car isn't burning any oil and emissions are spot on, you may want to consider installling a CEL-BOSS on the rear O2 sensor.

The previous owner had the Cat replaced, and since he worked at the dealership as a salesman I am going to assume it is OEM for now until he tells me otherwise. Not sure if h had the O2 sensors done at the same time but I will ask. I haven't noticed any drops in oil level or blueish exhaust smoke. I'll likely get a new rear sensor and put it on as they are quite cheap and then I'll post back in here if it resolved my CEL. Thanks.

Update: The CEL turned off on its own. After thinking about it some more I wonder if it was due to the cheap 87 gas I put into it a couple days before the CEL came on. Maybe it had to do a few tanks of 91 before the O2 readings leveled out back to normal.

is the code still stored? Some codes are "averaging" which requires xxx miles before the sampling is complete so that would explain why the CEL would turn off - but the code may still be stored. You can clear that with any OBD2 scanner (you can borrow from Autozone etc or buy a $30 Centec at Harbor Freight to keep in the car...)