P0420 keeps coming back

P0420. o2 sensor replaced, exhaust leak fixed, light is back on for the third time. What could it be?

 

Most likely a bad cat or two. I got this code last year and replaced both cats, code went away and I passed smog.

 

I guess the lift is going to fall through now.

 

Well the good news is, even if you live in CA you don't have to get OEM cats (stupid expensive $$$$)... Magnaflow makes good CARB approved ones for 1st gens for much less money, I got both installed for $600. Toyota wanted almost $3k.

If you don't need CARB approved ones then your options are even cheaper

 

VA has no emissions testing so imma let it be then and keep going. I love this truck but it’s so expensive

 

How does the system detect the issue anyway?

 

The bad thing about letting it keep going like that with bad cats is that the sensors picking up on the bad readings send faulty info to the ECU and the ECU compensates for that with more or less fuel, so you might start running too rich or too lean...which will eventually effect your MPGs, O2/air fuel sensors, and other parts of the engine over time.

I would just get some cheap cats and call it a day, although cheap cats might not last very long but at least they're cheap.

Or you could just cut the cats out entirely like some people do in non emissions states, but I think you'd still have the issues stated above. And your truck will sound and smell like crap.

 

So it’ll screw stuff up if I try and finesse the system?

 

It compares the output signals from the front and rear oxygen sensors and if it sees high millivoltage fluctuation activity out of the rear it knows the cat converter is toast.

 

Where is the front one located?

 

You mean like cutting out the cats? I know some people do it but the ECU is programmed to operate optimally based on correct readings from your sensors. If you remove the cats or run with bad cats, you aren't getting good readings from those sensors and the ECU will react accordingly.

There are ways to cheat the o2 sensors into thinking they are getting good readings, but I'm not that familiar with them

 

If I recall it's pretty close to the front of the truck right after the headers

 

Could it be that ones messed up and they didn’t replace it? He told me it wasn’t reading on the chart but then it was when they replaced the sensor. He could tell I was mad... maybe he just bs’d a story. What do you think?

 

OP, common problem for 1st gens as I learned when I had the same issue 2 months back. I found the replacement cat on AutoAnything for $200 including shipping. Took 2 weeks to arrive, but I'm not sure if it'll come sooner there in Virginia. I replaced the front cat and the sensors, and the code was gone. If you're interested in getting the one I got from them, the part number is 650511.

 

Usually it's the front that goes first, then the rear...so it's likely if you still have that code that it's the front cat

 

What about this
https://www.flowmastermufflers.com/...e=76&year1=2002&model=1037&submodel=0&engine=

 

So the front sensor is prob ok? And I just need to replace the cat that is right before the rear sensor?

 

P0420 means the sensors are functioning, the rear o2 sensor measures the mixture from the upstream and detects how the cat is functioning.

It's the cat 100%. At the dealer we NEVER throw o2 sensors at p0420, but online every one always suggests it for some reason, probably cost.

 

Using both o2 sensors. When the downstream o2 sensor gets a reading similar to the upstream it means the cat is no longer functioning the way it's supposed & generally the P0420 code comes up.

When you get P0420 it usually means o2s are working fine & the cat(s) are bad. Some people have replaced the downstream o2 sensor with it actually being bad but usually it's the cat(s).

I just replaced my front o2 sensor only for the second time yesterday & my truck is 17 yrs old. My rear still hasn't gave me trouble yet. Have never got P0420 yet in this truck. But if & when I do I'll replaced the rear o2 sensor first because it's cheaper (& because of age), then the cats if needed.

 

Sensor is probably ok, a bad o2 sensor throws a p0135/6 code...I had that recently. There are ways to check your cats, best way is to look through them at the material inside but you can't do that without taking them off. There are ways to diagnose the cats though...this video is a good tool
https://www.youtube.com/embed/kNXh1Lphzr4