Draven

My F-150 (2005, 4.6L XLT, 150k miles) had a check engine for a couple months now. I've erased it and it has kept coming back. P0420. I've done some reading. Apparently it's the catalytic converter, O2 sensors, or bad gas. I'm not sure if the cats are to blame as the previous owner swapped the stock exhaust for a dual pipe Flowmaster (40 classic I believe) setup. Ever since my latest fuel up I've been getting the worst gas mileage, but that code has come up before so I can rule out bad gas. I've replaced the fuel filter recently as well. Should I replace the O2 sensors? Or take it to a mechanic to check for cat problems? Thanks.

Mod (Ret.)

Before I'd replace the O2's, I'd get an exhaust shop (or comperable) to test the exhaust for plugged cats. At that mileage, it's very possible that the cats are indeed plugged or broken inside; the rear O2 sensors would be the ones that report this.

P0420 means bank 1 catalyst is reporting faulty, which is the side that has cylinder 1. The back O2's will set this code indicating a plugged catalytic converter, bad O2, exhaust leak, or exhaust damage.

If you can get under the truck, take a large stick or rubber mallet, and carefully strike that sides catalytic converter and listen for rattling, a sign that this catilytic converter is no good.

To check the O2 itself, it requires a scan type tool or oscilliscope that checks for a steady wave form that will bounce up and down, indicating the O2 is working correctly (detecting a lean or rich mixture), and sending that signal to the PCM so the fuel/air mixture can be adjusted.

HulluCorn

P0420 appeared shortly after leaving the dealership last week.
My plan is to change the plugs, fuel filter, and o2 sensors within the next two weeks.
In all honestly, I may have a small exhaust leak or plugged cat throwing the code. I can hear abnormal exhaust during cold idle.

I have:
2005 FX4 Ext Cab 5.5FT
5.4 3v
86K

Mod (Ret.)

Then why toss all those parts at it? You may still have the same code. If you have an exhaust leak or plugged cat (the truck would not be running well however), fix those first.

HulluCorn

Plugs are probably due -- my idle is not rough but it isn't clean either.
Motorcraft fuel filter was $5 and figured it was due.
Scanned live data and noticed one of the o2's readings changing rapidly.*
- This may be normal; however, if I can change normal maintenance for cheap, i figured why not.

Also, I have came across posts on Google that listed those parts as their solution to this code; even a gas cap swap helped someone.

My p0420 code is probably more than likely a exhaust leak, even if it is super small.

techrep

iTrader: (1)

I want to buy some Motorcraft Fuel filters for 5 bucks each.... where ?

HulluCorn

They may not be in stock. If you need a link i'll forward.

Search Amazon with the part number... Once you find the Amazon Prime seller...click used. These parts are not used, the packaging was damaged during transportation, and that is it.

I find that a lot of people know about this because these stores run out of stock all the time.

Hope this helps.

techrep

iTrader: (1)

Free shipping ??

HulluCorn

Yes
http://www.amazon.com/gp/offer-listi...condition=used

They are back up to $6.51.
If it is not urgent.. i would just keep checking every so often.

My goal is to get the best for the cheapest.

techrep

iTrader: (1)

Free shipping is on orders over $35... Not a $5 filter