I’ve had a p0420 code on my truck for a while which has prevented me from being able to register my truck, and I got burned on it the other day when I got a ticket (my fault, I know). I wanted to narrow down the possibilities by reading the output voltage of the o2 sensors to make sure that the sensors themselves were in working order but I can’t quite make sense of what I was getting back.
While cruising, the upstream sensors worked exactly as intended and as you’d expect. B2S2 was reading a consistent 0.6-0.8V, while B1S2 was jumping erratically from 0.1V to 0.7ishV, but not in a way that mirrors B2S1. While the car was at temp (~190) and cats at temp as well (~950-1200), I did a quick 2k rev test, and B1S2 held right around 0.1V consistently while all other sensors read as expected. My question is, should I replace the sensor, or just bite the bullet on the cat? I don’t really want to throw money at it until the problem is fixed, nor can I afford to. Thanks for any advice in advance.

 

How old is the sensor? If it's been replaced at some point in the past, what brand?

 

It was replaced at the same time as the cats by a shop near me, so I can’t verify the brand without removing it. The fact that it’s the same sensor that was replaced makes me think they put a cheap one on, and it failed already or triggered the PCM. Doesn’t bode well for the quality of the cats they installed if that’s the case.

 

Could try replacing the sensor with an NTK, or Denso sensor, see if anything changes...... I hate just firing the parts cannon though.

 

what truck and motor are you dealing with, assuming its a dual cat by the talk. If they were recently replaced you may be able to remove them and swap ports? This would help verify an issue with sensor, cat and wiring to sensor from PCM, I usually opt for tuning on the rear 02 sensors, its cheaper then trying to repair them usually, but if your in an inspection state, not sure

 

double post, delete

 

That’s very smart to switch the o2 sensors, I’ll have to keep that in mind should they fail in the future. I ended up replacing the plug and it cleared the code, I’m in an inspection county unfortunately

 

It was the sensor thankfully. Thank you for the advice.

 

I recommend trying to figure out the root cause of the P0420.

Typically, the catalytic converter is faulty when that code is set. However, the reason for the failure needs to be verified.

Make sure there are no misfires, vacuum leaks, MAF sensor contamination, or crankcase blow by, etc. You may want to ensure the engine is getting up to operating temperature: check coolant temperatures to make sure the thermostat isn't stuck open.

Swapping O2 sensors bank to bank (as suggested in earlier posts) is time consuming, but can verify O2 sensor operation.

And I also recommend checking the fuel quality. A bad tank of gasoline can cause drivability concerns and can damaged the catalytic converter(s).