P0420
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Fleet Mechanic
Joined: Mar 2007
Posts: 1,398
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From: Simi Valley, CA
P0420
Hey Guys,
Had the check engine light in the 2005 F150 (90,000 miles) come on. Got the code checked and its P0420. I know that has to do with the cat threshold levels etc. so i believe im looking to replace the o2 sensor, if im not mistaken.
How hard of a job is this? Ive done a decent amount of engine work and worked in a shop in high school. Just looking to see if that is the right thing to try first, replacing the sensor, before replacing the cat.
Also I know its the right bank, but if im not mistaken there is a before and after the cat o2 sensor, which do i replace? Both?
Brian
Had the check engine light in the 2005 F150 (90,000 miles) come on. Got the code checked and its P0420. I know that has to do with the cat threshold levels etc. so i believe im looking to replace the o2 sensor, if im not mistaken.
How hard of a job is this? Ive done a decent amount of engine work and worked in a shop in high school. Just looking to see if that is the right thing to try first, replacing the sensor, before replacing the cat.
Also I know its the right bank, but if im not mistaken there is a before and after the cat o2 sensor, which do i replace? Both?
Brian
Postmaster
Joined: Oct 2009
Posts: 3,432
Likes: 7
The P0420 code means your catalytic converter has failed. It does not tell you why it failed, which is really the question you should be asking.
The why is a more difficult question because we do not have all the details of what is really going on. 2005s do have some issues with the cats coming apart because Ford switched to the excellent design they had on the older models to the stupid but cheaper to make design found on 2005 and newer.
The most common causes for converter failure (don't argue with me on this, my expereince says this is true), is failure of the upstream O2 sensor. There is a reason Ford and the sensor manufacturers recommend replacing the sensors at 80,000 miles or so, and its not because they will sell more sensors that way, its because the vehicle has no way of verifying the sensors accuracy, and when they get older and get deposits on them, they tend to read as though the car is running lean, which causes the computer to add more fuel until it is satified with the reading. This results in a rich mixture, which over time results in carbon deposits, which in turn further reduces the accuracy of the sensors until the carbon deposits begin to adversely affect the converters themselves. When this happens, the catalytic converter can no longer store oxygen properly which is an important function of the converter. When this happens, the P0420 (or P0430 in the case of bank#2) code is set.
The solution is to first find and correct the cause of the converter failure. The test to measure the O2 sensors accuracy is both simple and complex. Its simple because any halfway descent scanner can do it. What you do is you measure the Long Term Fuel Trim to look for discrepancies that might indicate accuracy issues. Basically if the car is running properly, little or no changes to the average mixture should be needed, and the LTFT% will be within ± 3%. If it goes outside that range on a long term average, then the computer is attempting to compensate for something. The O2 sensors are maintenance items, so they are a good place to start. It is the upstream sensors you need to worry about, since they are involved with the fuel management.
If after replacing the sensors, the LTFT returns to the proper range, then you can replace the converters with good faith that your situation is resolved and you will have no further problems. If the LTFT has not returned to the proper range, then the underlying problem is still present and will still need to be corrected, otherwise your replacement converter will fail as well.
The why is a more difficult question because we do not have all the details of what is really going on. 2005s do have some issues with the cats coming apart because Ford switched to the excellent design they had on the older models to the stupid but cheaper to make design found on 2005 and newer.
The most common causes for converter failure (don't argue with me on this, my expereince says this is true), is failure of the upstream O2 sensor. There is a reason Ford and the sensor manufacturers recommend replacing the sensors at 80,000 miles or so, and its not because they will sell more sensors that way, its because the vehicle has no way of verifying the sensors accuracy, and when they get older and get deposits on them, they tend to read as though the car is running lean, which causes the computer to add more fuel until it is satified with the reading. This results in a rich mixture, which over time results in carbon deposits, which in turn further reduces the accuracy of the sensors until the carbon deposits begin to adversely affect the converters themselves. When this happens, the catalytic converter can no longer store oxygen properly which is an important function of the converter. When this happens, the P0420 (or P0430 in the case of bank#2) code is set.
The solution is to first find and correct the cause of the converter failure. The test to measure the O2 sensors accuracy is both simple and complex. Its simple because any halfway descent scanner can do it. What you do is you measure the Long Term Fuel Trim to look for discrepancies that might indicate accuracy issues. Basically if the car is running properly, little or no changes to the average mixture should be needed, and the LTFT% will be within ± 3%. If it goes outside that range on a long term average, then the computer is attempting to compensate for something. The O2 sensors are maintenance items, so they are a good place to start. It is the upstream sensors you need to worry about, since they are involved with the fuel management.
If after replacing the sensors, the LTFT returns to the proper range, then you can replace the converters with good faith that your situation is resolved and you will have no further problems. If the LTFT has not returned to the proper range, then the underlying problem is still present and will still need to be corrected, otherwise your replacement converter will fail as well.
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GingKnight
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