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Old 03-18-2011, 02:27 PM
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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
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Old 03-18-2011, 03:25 PM
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Hey Brian!

I don't have any experience with that, but I found a few things that might be helpful:

P0420 DTC Code - Catalyst System Efficiency Below Threshold (Bank 1)
https://www.ford-trucks.com/forums/6...ode-p0420.html
https://www.ford-trucks.com/forums/3...ode-p0420.html
P0420 Cut off cats? - F150online Forums

Seems like more often than not it's the cat and not the sensor.
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Old 03-18-2011, 03:29 PM
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Thanks Steve! i was hoping you would respond you are always a good resource! I couldnt find much in my searches. Thanks man!
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Old 03-18-2011, 03:49 PM
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Let us know what happens


Wait a minute...'05 F150? I thought you just bought a Dodge/w a 5.9 a couple of months ago.

You collecting trucks now?
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Old 03-18-2011, 04:01 PM
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haha, i am indeed! I sold the dodge and went back to a newer 05 F150. Needs change and i saw an opportunity to flip the dodge and make some money so i did! So i bought this 05 F150 with 90k miles for 13500 it has some problems but nothing i cant fix myself i believe.

I think im going to try replacing the o2 sensor. However it say that its the "right bank" or "bank 1" which side of the truck is this? i read it bank 1 is the side of cylinder 1, but i dont know which side cylinder 1 is on, so which side of the vehicle is bank 1 on? drivers or passenger?
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Old 03-18-2011, 04:09 PM
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I don't know offhand which side it is on a 5.4, but I do know that the cylinder bore that is farthest forward on the block is always designated as #1. Maybe you can tell by looking at it.
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Old 03-21-2011, 03:50 PM
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ok so i replaced the down stream sensor on the passenger side, cleared the code and drove around a bit and it came back on so it looks like it is the cat :-(
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Old 03-24-2011, 01:59 PM
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OK, so i took the truck back to my exhaust guy, i told him i replaced the downstream sensor, and the code came back he told me that its probably the cat in this case. However he told me that he didnt know of any CA legal aftermarket cat systems for an 05. He went through his book and said that I would have to go to the dealer to get the parts. The dealer told me that if i was going to get the new cats from them it would cost about $1900. However i talked to the guy at the parts desk and a mechanic and both told me to get a second opinion because a new cat at only 90k miles is pretty unlikely.

So, do i go and get a second opinion or are there CA legal cat systems out there that my guy simply isnt familiar with and that cost way less than a new dealership installed set up?

What is a second opinion really going to tell me? I know what the code is saying, its saying the downstream sensor is acting like the upstream sensor which means the cat isnt funtioning correctly, so do i spend another hundred bucks or so to get it looked at by someone else or spend that hundred with a couple other hundred and get a new cat system for the passenger side?
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Old 03-24-2011, 05:19 PM
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P0420 is catalytic system deficiency.

You changed the downstream sensor, maybe you should try swapping it with the upstream sensor? They are probably the same part, and the PCM uses the difference in values between the two.

But I think what you really need to do is get it into the dealer for a diagnostic. They can hook up and evaluate the reported value from each sensor, both banks, and tell you for sure if it is the cat.

I have also found that there are no aftermarket CA legal cats. It has to be OEM.
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