P0420 Code
Thread Starter
Teching In
Joined: Nov 2010
Posts: 46
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P0420 Code Recently my light came and so I scanned it and it read P0420 Catalytic Efficiency below Threshold. Many of you may have had this same problem.
I had both cats replaced when I bought the car a little over a year ago, so could it be an O2 Sensor? Would they come up a different code?
Thanks!
I had both cats replaced when I bought the car a little over a year ago, so could it be an O2 Sensor? Would they come up a different code?
Thanks!
The after cat o2 sensor is the only thing it could be if you just bought bRand new cats, the second o2 sensors job is to compare voltage to the voltage reading of the pre cat o2 sensor, if it doesn't see a drop in voltage it's the cat, or in your case the sensor, I would change that and go from there...there is a couple other things it could be
Low quality replacement converters, not runningin proper/optimal fuel control make for short cat life.
See it all the time.
Likely the PCM is right, the converters are on the weak end.
The O2's test for oxygen, the post cats check catalyst oxygen storage capacity.
See it all the time.
Likely the PCM is right, the converters are on the weak end.
The O2's test for oxygen, the post cats check catalyst oxygen storage capacity.
Thread Starter
Teching In
Joined: Nov 2010
Posts: 46
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I always run 93 Octane fuel, but I work at a parts store and it looks like both downstream and upstream sensors are the same, so I guess ill try that first.
If its the catalytic, I'm just going to redo the entire exhaust with headers and highflows.
If its the catalytic, I'm just going to redo the entire exhaust with headers and highflows.
FormerVendor
Joined: Dec 2011
Posts: 77
Likes: 0
From: Sandy, UT
Yeah, thats probably what it is; usually with that code it is the cat, not the O2 sensors. If the cat is good, the next step would be to replace the sensors, in this instance I always recommend replacing both at the same time if you know the cat is good. The pre-cat sensor could be causing it if its reading incorrectly. The computer uses the post cat sensor and compares it to the pre-cat sensors reading. If either one is wrong, it could potentially cause the code.
Rarely a sensor or computer problem, usually a bad cat, unless you have long tube headers and are not tuned for them.
- Jeremy
Rarely a sensor or computer problem, usually a bad cat, unless you have long tube headers and are not tuned for them.
- Jeremy
TECH Senior Member
Joined: Apr 2002
Posts: 6,080
Likes: 17
From: So.Cal.
The catalytic efficiency monitor test is designed to pinpoint cat failure by comparing response time between front O2 and rear O2, the test passes if the response time exceeds a minimum value;
if the rear O2 sensor fails, then the cat test either passes (due to response time exceeding minimum) or may not be run at all (if rear O2 DTC is present).
if the rear O2 sensor fails, then the cat test either passes (due to response time exceeding minimum) or may not be run at all (if rear O2 DTC is present).
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Hook up a scan tool and monitor the waveforms of all four O2 sensors. Fronts should be switching positive to negative in a nice wave at steady cruising and the rears should remain flat. If one of the rears is switching the same as the fronts it is most likely a bad cat on that bank. If you get drops in the signals or a jacked up signal it could be an O2 sensor or an air/fuel issue. Short term fuel trim will help you rule out a fuel or vacuum issue too.
If it turns out to be your new cats are toast you might want to get to the root problem of what is killing cats before just replacing them again or you could just take out another one. Could be burning a small amount of oil or coolant or running too rich or lean (fuel or vac leak). Could be a cracked weld in one of your new cats letting air in before the downstream O2 sensor. P0420 is a pretty vague code, it really just says there is a discrepancy between what signal the computer is expecting the rear O2 sensors to send and what they actually are sending but without more info there are too many variables to pinpoint the problem without guessing and throwing parts at it.
If it turns out to be your new cats are toast you might want to get to the root problem of what is killing cats before just replacing them again or you could just take out another one. Could be burning a small amount of oil or coolant or running too rich or lean (fuel or vac leak). Could be a cracked weld in one of your new cats letting air in before the downstream O2 sensor. P0420 is a pretty vague code, it really just says there is a discrepancy between what signal the computer is expecting the rear O2 sensors to send and what they actually are sending but without more info there are too many variables to pinpoint the problem without guessing and throwing parts at it.
Staging Lane
Joined: Jan 2013
Posts: 61
Likes: 0
From: Lyndon, KS
Alright, scanned a 20 minute drive with idling at either end. I got some screwy looking results.
This was immediately after refueling halfway through my trip home, car was warm, O2s warmed up.
At the initial idle, the upstream O2s showed a choppy but otherwise unremarkable pattern. The downstream showed a relatively stable low value <100mv. As soon as I started moving the bank 2 rear O2 went smoothly to an upper value near 800mv, and the bank 1 rear O2 started cycling similarly to the bank 1 O2's. During shifting to get up to speed, the bank 2 rear O2 would drop to the low value between shifts, then go back to the high value. At speed, the bank 1 rear O2 dropped back to a low steady value, and the bank 2 rear O2 stay at the higher value.
The other weirdness was the Fuel Trims. The LTFTs were between 3 to 16 (+) on bank 1, and -2 to almost 10 (+) on bank 2.
I'm thinking I have a slight air leak somewhere in the "new" cat area - and it must be feeding back to the O2s. I just filled up before the log and got ~232 miles out of 12 gallons, most of which was driving a steady 65-75 mpg.
This was immediately after refueling halfway through my trip home, car was warm, O2s warmed up.
At the initial idle, the upstream O2s showed a choppy but otherwise unremarkable pattern. The downstream showed a relatively stable low value <100mv. As soon as I started moving the bank 2 rear O2 went smoothly to an upper value near 800mv, and the bank 1 rear O2 started cycling similarly to the bank 1 O2's. During shifting to get up to speed, the bank 2 rear O2 would drop to the low value between shifts, then go back to the high value. At speed, the bank 1 rear O2 dropped back to a low steady value, and the bank 2 rear O2 stay at the higher value.
The other weirdness was the Fuel Trims. The LTFTs were between 3 to 16 (+) on bank 1, and -2 to almost 10 (+) on bank 2.
I'm thinking I have a slight air leak somewhere in the "new" cat area - and it must be feeding back to the O2s. I just filled up before the log and got ~232 miles out of 12 gallons, most of which was driving a steady 65-75 mpg.
Last edited by Jonota; Jan 6, 2013 at 11:18 PM. Reason: Initial conditions
On The Tree
Joined: Apr 2007
Posts: 165
Likes: 0
From: Bloomington, MN
Some brand new cats will throw a code because they are not capable of the efficiency that the P0420 codes demand. Hi-flow cats are big offenders of this. Before you go throwing parts at it, figure out what it really is. Take the two post-cat sensors and swap them - reset code. If the light comes on again and you get a P0420 again, then its likely the drivers side cat. If it comes back as a P0421 code (pass side) then you likely have a bad sensor.
Another option is to drill out a spark plug anti-fowler and space out the O2 sensor. The post cat O2 wont impact performance, itll just trigger the light. You could also tune it out, but that costs a lot more than an anti-fowler.
Another option is to drill out a spark plug anti-fowler and space out the O2 sensor. The post cat O2 wont impact performance, itll just trigger the light. You could also tune it out, but that costs a lot more than an anti-fowler.
Staging Lane
Joined: Jan 2013
Posts: 61
Likes: 0
From: Lyndon, KS
I'm going to put it up on my lift and check everything out when I get back home. Had to leave the state for a few days for family stuff. I'm betting I have an air leak due to the LTFT being affected - which don't look at the post-cat O2. After I've checked out any leak paths, I'll swap the sensors as well to see how that works. I have HPTuners, so eventually it will be tuned out. However, I definitely want to solve the issue giving me 19mpg hwy!
Staging Lane
Joined: Jan 2013
Posts: 61
Likes: 0
From: Lyndon, KS
Put it up on the lift and found the driver's side union between the down pipe and Y-pipe loose. I haven't scanned to verify that was my problem, but as the O2 sensor is RIGHT by this union I imagine it had some effect. I didn't swap sensors at this time just to see what effect tightening that up had. Inspecting the "new" catalytic converters, the drivers side looked like the "bullet" style highflow cats, so that also might be playing a part. However, when I scanned before, the after cat mv reading simply stayed high, not cycling with the before cat.
Hook up a scan tool and monitor the waveforms of all four O2 sensors. Fronts should be switching positive to negative in a nice wave at steady cruising and the rears should remain flat. If one of the rears is switching the same as the fronts it is most likely a bad cat on that bank. If you get drops in the signals or a jacked up signal it could be an O2 sensor or an air/fuel issue. Short term fuel trim will help you rule out a fuel or vacuum issue too.
If it turns out to be your new cats are toast you might want to get to the root problem of what is killing cats before just replacing them again or you could just take out another one. Could be burning a small amount of oil or coolant or running too rich or lean (fuel or vac leak). Could be a cracked weld in one of your new cats letting air in before the downstream O2 sensor. P0420 is a pretty vague code, it really just says there is a discrepancy between what signal the computer is expecting the rear O2 sensors to send and what they actually are sending but without more info there are too many variables to pinpoint the problem without guessing and throwing parts at it.
If it turns out to be your new cats are toast you might want to get to the root problem of what is killing cats before just replacing them again or you could just take out another one. Could be burning a small amount of oil or coolant or running too rich or lean (fuel or vac leak). Could be a cracked weld in one of your new cats letting air in before the downstream O2 sensor. P0420 is a pretty vague code, it really just says there is a discrepancy between what signal the computer is expecting the rear O2 sensors to send and what they actually are sending but without more info there are too many variables to pinpoint the problem without guessing and throwing parts at it.
Put it up on the lift and found the driver's side union between the down pipe and Y-pipe loose. I haven't scanned to verify that was my problem, but as the O2 sensor is RIGHT by this union I imagine it had some effect. I didn't swap sensors at this time just to see what effect tightening that up had. Inspecting the "new" catalytic converters, the drivers side looked like the "bullet" style highflow cats, so that also might be playing a part. However, when I scanned before, the after cat mv reading simply stayed high, not cycling with the before cat.
Staging Lane
Joined: Jan 2013
Posts: 61
Likes: 0
From: Lyndon, KS
I haven't scanned to confirm, but I'm hopeful that that was the problem. It won't matter for too long anyway, now that I've gotten a look at that Y-pipe it's GOT TO GO. Holy moly is that thing terrible (the stock Y). The other 2 4th gens I had had full aftermarket exhaust already when I got them so this was somewhat of a shocker.
I definitely know it's not the cats - the downstream waveform was not a repeat of the upstream - it was solid. So that's somewhat of a relief. Now I just need to solve my air leak or sensor issue ::crossing fingers::
I definitely know it's not the cats - the downstream waveform was not a repeat of the upstream - it was solid. So that's somewhat of a relief. Now I just need to solve my air leak or sensor issue ::crossing fingers::
Staging Lane
Joined: Jan 2013
Posts: 61
Likes: 0
From: Lyndon, KS
Well, I can't say that I have it "fixed". Interestingly enough, there was some progress today.
I first tackled the exhaust leak. I did the 'paper' trick at the manifold to cat union and it was blowing air. So ordered new LT's and Y-pipe (Pacesetter). Install was smooth as silk, programmed out the associated codes, and my mileage went up to around 23 avg (so +4mpg). I still wasn't happy - have been debating new O2 sensors, did a round of seafoam through the PCV port as well.
Today for giggles as I was torquing my header bolts again, I decided to check my intake manifold torque per another LS1tech poster on my own thread. To my dismay, out of 10 intake manifold bolts ONE was at torque, and most took over 2 full turns to get to tightness. The manifold visibly moved down - I am now hopeful that will get me the last few MPG that I'm missing.
I first tackled the exhaust leak. I did the 'paper' trick at the manifold to cat union and it was blowing air. So ordered new LT's and Y-pipe (Pacesetter). Install was smooth as silk, programmed out the associated codes, and my mileage went up to around 23 avg (so +4mpg). I still wasn't happy - have been debating new O2 sensors, did a round of seafoam through the PCV port as well.
Today for giggles as I was torquing my header bolts again, I decided to check my intake manifold torque per another LS1tech poster on my own thread. To my dismay, out of 10 intake manifold bolts ONE was at torque, and most took over 2 full turns to get to tightness. The manifold visibly moved down - I am now hopeful that will get me the last few MPG that I'm missing.
12 Second Club
Joined: Sep 2005
Posts: 4,273
Likes: 0
From: Bucks County, Pa.
If you put 49 state catcons (EVEN brand new, Magnaflow stock fit replacements) on a Cali/LEV car (they shipped these to MANY other states besides the 'Republik'), they will eventually set this code, sometimes after not very long (less than half a year in my case).
This is because the ECM for these cars sets the threshold MUCH tighter than the 49 state specced cars' ECMs.
Ask me how I know.
This is because the ECM for these cars sets the threshold MUCH tighter than the 49 state specced cars' ECMs.
Ask me how I know.
I had an issue several years ago, both 02 sensors triggered a code at the same time. The dealership thought it was the MAF so I replaced that and haven't had an issue since.