p0420 help

[FinalBreath05]

Hey Matthew,

From the service manual, short and long term fuel trims should be "Near 0%".
The HO2S sensors for both banks, sensors 1 and 2, just state they should be "10 - 1000mV and varying". In my truck, both "sensor 1"s pulse pretty regularly from about 65mV to under 900mV. Sometimes they are in phase, sometimes they aren't, sometimes they are the same width, sometimes they aren't. My "sensor 2"s don't fluctuate that much (at least at idle) and tend to be in the 600-700mV range. Don't take my sensor 2 values as what they *should* be, though, as my LTFT at idle aren't zero, they are between -10 and -14% currently.

Thank you, I'll check it out.

 

[swathdiver]

Hey Matthew,

From the service manual, short and long term fuel trims should be "Near 0%".
The HO2S sensors for both banks, sensors 1 and 2, just state they should be "10 - 1000mV and varying". In my truck, both "sensor 1"s pulse pretty regularly from about 65mV to under 900mV. Sometimes they are in phase, sometimes they aren't, sometimes they are the same width, sometimes they aren't. My "sensor 2"s don't fluctuate that much (at least at idle) and tend to be in the 600-700mV range. Don't take my sensor 2 values as what they *should* be, though, as my LTFT at idle aren't zero, they are between -10 and -14% currently.

Ditto for mine.

 

[Doubeleive]

A crawled under my truck to swap downstream sensors from side to side as I'd been getting a P0430 daily. Noticed that right side cat was not original and that it was discolored and the sensors wouldn't budge with just a wrench on them. I concluded from those observations that the cat was really at fault. Lord willing, by the end of the week I'll have two new cats underneath and will see how she drives before dropping $200 on 4 new GM O2 sensors.

I do remember now having a constant supply of new O2 sensors for the Grand Nationals, the additives and race fuel quickly degraded those old 1 wire sensors and the cars would slow down until we put a fresh one in. The GNXs had a differently calibrated O2 sensor and we used those.

you just need a bigger wrench, I did all 4 of mine for $170 and 2 of those were 10-15 more because they were the pro grade (rockauto)

 

[Scottydoggs]

Ditto for mine.

short term trims are the correction for the long term. at idle you like to see then at 0 or bouncing -/+ real low to zero, like -0.8 to +0.8 and touching zero between. and they should always lock in at zero when wot, the rest of the time they dance all over.

long term is much more important on a scan.

these trucks seem to come with a lean ass stock tune. mine was all high pos trim numbers till i started tuning it. like +8. ive got most ltft cells to zero, still need to tune the maf in wot some more. im still hitting +3 and +4's up there in higher rpms. like 4000 up.

 

[FinalBreath05]

Here's what I found on my scan.

Cold STFT Bank 1 0-2 at idle. Bank 2 0-2 at idle. LTFT Bank 1 9 at idle. Bank 2 10 at idle. MAF 6.2 MAP 11.2. All of Bank 1 has misfires in the history. Once the truck warmed up STFT Bank 1 -5 to 0. Bank 2 -4 to 0. LTFT Bank 1 was 5 and Bank 2 was 6.

 

[Scottydoggs]

Here's what I found on my scan.

Cold STFT Bank 1 0-2 at idle. Bank 2 0-2 at idle. LTFT Bank 1 9 at idle. Bank 2 10 at idle. MAF 6.2 MAP 11.2. All of Bank 1 has misfires in the history. Once the truck warmed up STFT Bank 1 -5 to 0. Bank 2 -4 to 0. LTFT Bank 1 was 5 and Bank 2 was 6.

see how the stft goes neg, thats to counter the long term being pos.

the trims will be richer generally on a cold start, fuel injection has no choke, so it adds more fuel to make it idle higher. so the trims get richer for a few minutes, as the idle drops it leans back out to normal.

that all looks pretty much like i had stock on mine. if you drive it and scan then get in the gas you'll see the long term go higher with a pos number.

set that up to scan for knock retard when you drive it too.

misfires could be it needing a tune up if its never been done or if its time again, not knowing your miles. or its possible that that one cat is clogging up on ya, not knowing your miles, thats a shot in the dark.

 

[FinalBreath05]

see how the stft goes neg, thats to counter the long term being pos.

the trims will be richer generally on a cold start, fuel injection has no choke, so it adds more fuel to make it idle higher. so the trims get richer for a few minutes, as the idle drops it leans back out to normal.

that all looks pretty much like i had stock on mine. if you drive it and scan then get in the gas you'll see the long term go higher with a pos number.

set that up to scan for knock retard when you drive it too.

misfires could be it needing a tune up if its never been done or if its time again, not knowing your miles. or its possible that that one cat is clogging up on ya, not knowing your miles, thats a shot in the dark.

Tune up was just done about 5k miles ago. I replaced plugs, wires and coil packs. The side with the misfires is the side that I have the P0420 for.

 

[Scottydoggs]

Tune up was just done about 5k miles ago. I replaced plugs, wires and coil packs. The side with the misfires is the side that I have the P0420 for.

that leads me to believe its time for new cats. id do both, if one is bad the other is not far behind it.

 

[FinalBreath05]

that leads me to believe its time for new cats. id do both, if one is bad the other is not far behind it.

That's what I'm thinking. Can I use a spark plug non fouler to trick the rear o2 sensors?

 

[Scottydoggs]

That's what I'm thinking. Can I use a spark plug non fouler to trick the rear o2 sensors?

no, thats for when you install a high flow cat and get a code.

clogged cats need to be replaced. the cat is causing the misfires, and the 420 code.

new cats would add back some power too i bet, ones clearly not flowing as well as it could be.

think Beverly hills cop, bananas in the tail pipe, same trick only its the cat being clogged up.