p0420 after sensor replacement

[jbarry]

I've been reading a lot of posts/sites about the p0420 code but was hoping for some direction.

I've been fighting a different code p0171 for a while with varying degrees of success. Finally I decided since I've got about 150k on my 99 limited v6 4wd, that I'd replace the a/f sensor and the o2/sensor.

That was impossible to do on my own since they were so rusted on, I finally broke down and took it to a muffler shop. They got everything on and installed, I took it for an emissions test, passed no problems, then I get home and I've got the cel on with a code p0420.

I haven't yet had a chance to check the freeze frame data or look at the waveforms from the sensor, those are on my to do list but I'm wondering if this might be a bad install job. I have them the Denso parts I bought, 234-9002 a/f and 234-4153 o2.

Is it possible that these were installed incorrectly or in reverse which is why I'm getting the code now? If so, are the sensors labeled so of I take one off I can see the part number?

I cleaned the MAF before it went to the muffler shop. I'm trying to do all I can before having to bring it in some place.

Thanks in advance for any advice.

 

[TheDurk]

The A/Fs and O2s have different plugs so it would take a persistent dufus to install those in the wrong place. You would also get more codes than that, I would think. However, any leak in the exhaust system can provoke a P0420 so if they did not clean the mating surfaces well or get them tight enough, that could be the problem. You said they were pretty rusted. If they tried to reuse old rusted nuts or the studs were corroded, that could be the cause of a leak and thus the code.

Anyway, that's my first thought.

The sensors don't bear the part number, because many part numbers differ only by the length of the wire. But the plug protects you on that.

 

[Robb235]

Check out my writeup for building your own rear O2 simulator. I had the P0420 code forever, and the $5 RadioShack simulator fixed it.

 

[jbarry]

The A/Fs and O2s have different plugs so it would take a persistent dufus to install those in the wrong place. You would also get more codes than that, I would think. However, any leak in the exhaust system can provoke a P0420 so if they did not clean the mating surfaces well or get them tight enough, that could be the problem. You said they were pretty rusted. If they tried to reuse old rusted nuts or the studs were corroded, that could be the cause of a leak and thus the code.

Anyway, that's my first thought.

The sensors don't bear the part number, because many part numbers differ only by the length of the wire. But the plug protects you on that.

I checked the waveforms and they behave like I think they are supposed to, front is oscillating rear is basically a straight line.

I crawled under the truck and looked at the read sensor. They welded on a new part to hold it but it looks like it's on there nice and flush. The emissions guys did a visual inspection of the exhaust system and saw no leaks.

I guess I should just go down the whole checklist from the fsm?

 

[Robb235]

I checked the waveforms and they behave like I think they are supposed to, front is oscillating rear is basically a straight line.

That sounds backwards. If you have the California emissions (two catalytic converters), then you have an Air Fuel Ratio sensor, which is a wideband sensor. The wideband should be a constant, steady voltage.

If you have the single catalytic converter style exhaust, then you have the narrowband up front. It should switch rapidly between high and low voltage.

The rear sensor is narrowband, so it should be oscillating high and low no matter which year 3rd gen you have. If it's not switching back and forth, then something's wrong.

 

[ThorInc]

.......so it would take a persistent dufus to install those in the wrong place............

That's priceless!!! :bravo:

 

[jbarry]

That sounds backwards. If you have the California emissions (two catalytic converters), then you have an Air Fuel Ratio sensor, which is a wideband sensor. The wideband should be a constant, steady voltage.

If you have the single catalytic converter style exhaust, then you have the narrowband up front. It should switch rapidly between high and low voltage.

The rear sensor is narrowband, so it should be oscillating high and low no matter which year 3rd gen you have. If it's not switching back and forth, then something's wrong.

It's also possibly my own misunderstand, I probably had then backward in my head.

Bank 1 sensor 1 is flat
Bank 1 sensor 2 is oscillating.