Code 420, Need help to diagnose
Cargo Master
Joined: Aug 2003
Posts: 3,263
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From: BC Canada
Positive LTFT means the map was initially lean and the LTFT's were modified to add fuel.
Maybe this will help..
What are fuel trims all about?
... Philip
Maybe this will help..
What are fuel trims all about?
... Philip
Last edited by aquanaut20; Dec 18, 2012 at 06:56 PM. Reason: Computer hung during save....
Junior User
Joined: Sep 2012
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unless you computer was cleared by either disconnecting the battery or by a scan tool, you might be able to view the freeze frame data which will tell you how it was running at the time the code was sent. i say might cause i don't know if the computer deletes it itself when it turns the light out after a recheck/drive cycles or if that tool has the option
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Update: I replaced the upstream O2 sensors today. After I replaced them I drove it around the block and checked the data. The LTFT for banks 1 and 2 were positive values of 3.1 and .08. My check engine light was off so I thought I had it fixed.
Then I drove to dinner and on the way home the check engine light came back on. I pulled the same code as before, P0420. I ran the data and the LTFT increased to positive 7.0 and 3.8. Now I'm kind of stumped as to what is causing the problem. Any thoughts on where I should try next? Thanks.
Then I drove to dinner and on the way home the check engine light came back on. I pulled the same code as before, P0420. I ran the data and the LTFT increased to positive 7.0 and 3.8. Now I'm kind of stumped as to what is causing the problem. Any thoughts on where I should try next? Thanks.
Postmaster
Joined: Oct 2009
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Under what conditions is the fuel trim recorded? At idle, at 2500 rpm? What conditions the fuel trims you are viewing are under is a related factor. Vacuum leaks, dirty MAF, worn spark plugs, incomplete combustion, all of these things affect the fuel trim.
When was the last time this car had a full tune up?
When was the last time this car had a full tune up?
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Freshman User
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The fuel trim tests were performed at idle which was about 750 rpm. If I perform the test at other rpm ranges what should I be looking for?
I purchased the car in march and the P0420 code was present when I purchased it. I then performed a full tune up at that time. I have since put about 12k miles on it. The tune up included new spark plugs, fuel filter, air filter, pcv valve, and serpentine belt. Then more recently I replaced the radiator and flushed the cooling system. I also have now replaced all 4 O2 sensors.
I pulled the MAF earlier tonight and the wires did have a light dirt film on them, but I'm not sure what's normal so I can't say if it was excessively dirty or not. I was going to clean it but am out of MAF cleaner so I might try that tomorrow.
I also tried to check for vacuum leaks. From what I can tell, everything appears to be connected properly and I could not find any leaks. That was just visually inspecting it and running my hands over the lines.
I purchased the car in march and the P0420 code was present when I purchased it. I then performed a full tune up at that time. I have since put about 12k miles on it. The tune up included new spark plugs, fuel filter, air filter, pcv valve, and serpentine belt. Then more recently I replaced the radiator and flushed the cooling system. I also have now replaced all 4 O2 sensors.
I pulled the MAF earlier tonight and the wires did have a light dirt film on them, but I'm not sure what's normal so I can't say if it was excessively dirty or not. I was going to clean it but am out of MAF cleaner so I might try that tomorrow.
I also tried to check for vacuum leaks. From what I can tell, everything appears to be connected properly and I could not find any leaks. That was just visually inspecting it and running my hands over the lines.
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ok we know what the trims are, granted there a tad high since 0 is ideal. i just changed all my intake gaskets (upper and lower) and mine are in the same ball park with no p0420 code (but got a stubborn evap leak). since you have access to live data, what do the readings for the o2 sensors look like? upstream bouncing off the floor and ceiling so to say and down stream holding steady, or 3 of the 4 if not all 4 bouncing off the floor and ceiling. (i'm assuming that you have a dual exhaust since you got 4 sensors and I don't think the codes will tell you cat bank 1 or 2)
Postmaster
Joined: Oct 2009
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LTFT% that are higher at idle than when tested at higher RPMs points to vacuum leaks. LTFT% that is high all the time suggests dirty or contaminated MAF. If the LTFT% climbs as RPMs and load increases, then you may have a fuel delivery problem.
On the MAF any dirt on the element is too much. Clean it with a purpose made cleaner and see if that doesn't improve the LTFT% (and throttle response).
Vacuum leaks can occur as the upper and lower intake gaskets harden and shrink with age. These leaks can be serious and yet difficult to track down. Leaks in the EVAP lines can also cause issues.
On the MAF any dirt on the element is too much. Clean it with a purpose made cleaner and see if that doesn't improve the LTFT% (and throttle response).
Vacuum leaks can occur as the upper and lower intake gaskets harden and shrink with age. These leaks can be serious and yet difficult to track down. Leaks in the EVAP lines can also cause issues.
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I cleaned the MAF and throttle body today with the correct cleaner for both. I cleared the codes and drove it around for a little bit and tried to get different types of driving conditions. When I idle at a stop light my LTFT were positive 5.4 and 3.9. While driving they fluctuated and both banks would float between a positive 7 - 10. When I floored it the numbers would go as high as 14 or 15.
Additionally the readings for O2S11, O2S12, and O2S21 fluctuated from 0 to .8. However O2S22 stayed pretty constant around .680. The scanner gives me lots of other data as well, but I'm not quite sure how to interpret most of it, so it there is other data I should look at also just let me know.
Lastly, I'm not sure if this helps, but during the test drive the check engine light did not come on. It seemed to stay on more in the summer and now that the cold weather is here it is staying off more.
Additionally the readings for O2S11, O2S12, and O2S21 fluctuated from 0 to .8. However O2S22 stayed pretty constant around .680. The scanner gives me lots of other data as well, but I'm not quite sure how to interpret most of it, so it there is other data I should look at also just let me know.
Lastly, I'm not sure if this helps, but during the test drive the check engine light did not come on. It seemed to stay on more in the summer and now that the cold weather is here it is staying off more.
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o2s11 (bank 1 sensor 1) and O2S21 (bank 2 sensor 1) should bounce like that but O2S12 (bank 1 sensor 2) should hold steady like O2S22. Since O2S12 bounced around, I would have to say that that cat isn't working. Bank 1 sensor 2 would be the sensor right after the cat on the pass side (4x4 or rear wheel drive). I would be buying a cat after seeing that.
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Ok, that helps. It will be much cheaper and easier to replace the one bad cat instead of all three.
My only question is what about the fuel trim levels. I would hate to replace the cat only to have it go bad again.
From what Khan said, my fuel trims are acting like a lack of fuel supply. If that's the case then I'm wondering if the two problems are unrelated. Compensating for a lack of fuel supply wouldn't cause it to run rich and destroy the cat. Is that correct?
The only other thing I could think of would be if the fuel pressure sensor is bad and reading an incorrect lack of fuel supply. But I wouldn't know how to test that. Maybe I'm missing something here though.
Thanks for everyone's help so far! This has really helped me narrow down the problem.
My only question is what about the fuel trim levels. I would hate to replace the cat only to have it go bad again.
From what Khan said, my fuel trims are acting like a lack of fuel supply. If that's the case then I'm wondering if the two problems are unrelated. Compensating for a lack of fuel supply wouldn't cause it to run rich and destroy the cat. Is that correct?
The only other thing I could think of would be if the fuel pressure sensor is bad and reading an incorrect lack of fuel supply. But I wouldn't know how to test that. Maybe I'm missing something here though.
Thanks for everyone's help so far! This has really helped me narrow down the problem.
Junior User
Joined: Sep 2012
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Ok, that helps. It will be much cheaper and easier to replace the one bad cat instead of all three.
My only question is what about the fuel trim levels. I would hate to replace the cat only to have it go bad again.
From what Khan said, my fuel trims are acting like a lack of fuel supply. If that's the case then I'm wondering if the two problems are unrelated. Compensating for a lack of fuel supply wouldn't cause it to run rich and destroy the cat. Is that correct?
The only other thing I could think of would be if the fuel pressure sensor is bad and reading an incorrect lack of fuel supply. But I wouldn't know how to test that. Maybe I'm missing something here though.
Thanks for everyone's help so far! This has really helped me narrow down the problem.
My only question is what about the fuel trim levels. I would hate to replace the cat only to have it go bad again.
From what Khan said, my fuel trims are acting like a lack of fuel supply. If that's the case then I'm wondering if the two problems are unrelated. Compensating for a lack of fuel supply wouldn't cause it to run rich and destroy the cat. Is that correct?
The only other thing I could think of would be if the fuel pressure sensor is bad and reading an incorrect lack of fuel supply. But I wouldn't know how to test that. Maybe I'm missing something here though.
Thanks for everyone's help so far! This has really helped me narrow down the problem.
if you still want to check your fuel psi, there might be a test port on your fuel rail that has a blue cap on it. That way all you need to do is attach a fuel psi gauge to it.
Postmaster
Joined: Oct 2009
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I cleaned the MAF and throttle body today with the correct cleaner for both. I cleared the codes and drove it around for a little bit and tried to get different types of driving conditions. When I idle at a stop light my LTFT were positive 5.4 and 3.9. While driving they fluctuated and both banks would float between a positive 7 - 10. When I floored it the numbers would go as high as 14 or 15.
Additionally the readings for O2S11, O2S12, and O2S21 fluctuated from 0 to .8. However O2S22 stayed pretty constant around .680. The scanner gives me lots of other data as well, but I'm not quite sure how to interpret most of it, so it there is other data I should look at also just let me know.
Lastly, I'm not sure if this helps, but during the test drive the check engine light did not come on. It seemed to stay on more in the summer and now that the cold weather is here it is staying off more.
Additionally the readings for O2S11, O2S12, and O2S21 fluctuated from 0 to .8. However O2S22 stayed pretty constant around .680. The scanner gives me lots of other data as well, but I'm not quite sure how to interpret most of it, so it there is other data I should look at also just let me know.
Lastly, I'm not sure if this helps, but during the test drive the check engine light did not come on. It seemed to stay on more in the summer and now that the cold weather is here it is staying off more.
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Freshman User
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I know this is an old post, but hey I got busy and I still have this problem. I was able to read the data when the code was set and was a little surprised at what I found. It showed lots of data but below is the fuel trim data it showed when the code was set. Does anyone have an idea what this means. I was pretty surprised to see such large negative numbers.
ST FTRM 1, 0.8%
LT FTRM 1, 5.4%
ST FTRM 2, 0.0%
LT FTRM 2, 5.4%
ST FTRM 3, -100.6%
LT FTRM 3, -100.6%
LT FTRM 4, -100.6%
ST FTRM 4, -100.6%
ST FTRM 1, 0.8%
LT FTRM 1, 5.4%
ST FTRM 2, 0.0%
LT FTRM 2, 5.4%
ST FTRM 3, -100.6%
LT FTRM 3, -100.6%
LT FTRM 4, -100.6%
ST FTRM 4, -100.6%