P0420 Sets At idle only
Thread Starter
Launching!
Joined: Jul 2011
Posts: 206
Likes: 1
From: Indianapolis
P0420 Sets At idle only Having a bit of a problem.. I had a bad MAF a few months back, didn't code but was stair stepping between 2,000-2,500 RPM pinned it down to a bad MAF.. well it also started setting a p0420 about the same time, but after i replaced the maf and cleared the codes It never came back until yesterday on my way home from work.
So once i got to work this morning I got out my scanner and checked the codes, P0420 set I look at the freeze frame data It set at idle and my LTFT B1 and B2 were 120's WTF? so i clear them out let it run at idle noting
STFT 0 LTFT 0 for 35 min, take it out on a test drive everything stays 0 all normal..
I can't figure this **** out and its making me crazy.. I drove the car for over an hour and noting 10 key cycles even and didn't set a single code..
on my way home from work come off the highway first light i stop at im waiting to turn watching the light, it kicks on just sitting there I don't understand this one at all... and im going to guess when i get to work tomorrow the LTFT is going to be crazy again
So once i got to work this morning I got out my scanner and checked the codes, P0420 set I look at the freeze frame data It set at idle and my LTFT B1 and B2 were 120's WTF? so i clear them out let it run at idle noting
STFT 0 LTFT 0 for 35 min, take it out on a test drive everything stays 0 all normal..
I can't figure this **** out and its making me crazy.. I drove the car for over an hour and noting 10 key cycles even and didn't set a single code..
on my way home from work come off the highway first light i stop at im waiting to turn watching the light, it kicks on just sitting there I don't understand this one at all... and im going to guess when i get to work tomorrow the LTFT is going to be crazy again
Thread Starter
Launching!
Joined: Jul 2011
Posts: 206
Likes: 1
From: Indianapolis
Scanning with a Snap-on VERUS The problem I'm Having is catching it while the scanner is on it.. The car is completely stock, seems like i can drive the car around forever then out of the blue it will set this code. i had put almost 7k on the car since i last seen this code (after replacing the MAF)..
about 3 weeks ago I took a 3.5 hr one way trip to the very top of Indiana, there and back no MIL the whole time..
I'm not sure if I am missing something here or not, and my main concern is why does the LTFT fly into the 120's out of nowhere? and just sitting at idle.. could it be because of the cat? but then why both banks, if only my B1 cat is messed up..?
I can check and see if my scanner will let me upload the data stream to here,
about 3 weeks ago I took a 3.5 hr one way trip to the very top of Indiana, there and back no MIL the whole time..
I'm not sure if I am missing something here or not, and my main concern is why does the LTFT fly into the 120's out of nowhere? and just sitting at idle.. could it be because of the cat? but then why both banks, if only my B1 cat is messed up..?
I can check and see if my scanner will let me upload the data stream to here,
The CAT code and the LTFTs are most likely completely seperate issues. I've seen other makes use fuel trims to try and force post cat O2s to move, but not GMs.
The P0420 code is most likely to set at slow speeds, or idle conditions. CATS work of temp. While cruising, the exhaust temp is a bit higher, and the cat is less likely to "turn off". At idle, or very low engine speeds, the CAT is MORE LIKELY to "turn off". So, not getting the code on a ling road trip, yet getting it in stop and go traffic is completely normal.
P0420 and 430 codes are very specific. To even be able to set these codes, the PCM has to pass the O2 monitors, assuring that both upstream and downstream O2s are functioning correctly. Then, based off the swing of the upstream, compared to the downstream (lack of swing, sustained high voltage indicating oxygen storage) and it's pattern of following the upstream on decel (delayed drop to low voltage) the computer can judge the efficiency of the cat.
Here's a scope shot of upstream and down stream O2s with a bad cat.
And here's the same with a good cat.
In other words, you most likely have a bad CAT.
The P0420 code is most likely to set at slow speeds, or idle conditions. CATS work of temp. While cruising, the exhaust temp is a bit higher, and the cat is less likely to "turn off". At idle, or very low engine speeds, the CAT is MORE LIKELY to "turn off". So, not getting the code on a ling road trip, yet getting it in stop and go traffic is completely normal.
P0420 and 430 codes are very specific. To even be able to set these codes, the PCM has to pass the O2 monitors, assuring that both upstream and downstream O2s are functioning correctly. Then, based off the swing of the upstream, compared to the downstream (lack of swing, sustained high voltage indicating oxygen storage) and it's pattern of following the upstream on decel (delayed drop to low voltage) the computer can judge the efficiency of the cat.
Here's a scope shot of upstream and down stream O2s with a bad cat.
And here's the same with a good cat.
In other words, you most likely have a bad CAT.
Thread Starter
Launching!
Joined: Jul 2011
Posts: 206
Likes: 1
From: Indianapolis
ok that i can except, but what is the deal with the LTFT going crazy? the car runs fine no black smoke or anything like that.. and its only the LT not the ST that spikes like this.. I'm sure the cat is bad anyway i can watch the live data and at idle the down stream does kinda follow the up stream a bit, not exactly but pretty close..
LTFTs will not go past 25 without setting a mixture code.
More likely this (LTFT=120) is spurious data over the OBD-II
(or the scan tool mis-acquiring the stream or mis-assigning
the data).
More likely this (LTFT=120) is spurious data over the OBD-II
(or the scan tool mis-acquiring the stream or mis-assigning
the data).
