I have an 04 5.4 3v FX4 that is causing me some grief.

I replaced my cat's about a year ago, and over the weekend my CEL came on. I pulled the codes and got P0420. I know that is bank 1 catalyst. I couldn't believe that it went bad that fast, so I did some logging of the O2 sensor data and fuel trims. I am not sure exactly how to interpret the data though.

I logged the values every second, and I can see my bank 2 front O2 going up and down in a fairly identifiable pattern, the rear sensor on bank too stays high for the most part moves a little bit but between .55 and .8

On bank 1 the front O2 is all over the place hangs at a high voltage for a while, then goes way low, then sits in the middle. The rear on bank 1 is also all over the place, but can be low voltage while the front is high, but not anywhere near as stable as the bank 2 sensors. In some cases the front and rear o2's are the same value on bank 1.

Long term fuel trims on bank one ranged from -2 to +15, short terms from -11 to +21

Long term on bank 2 were from -5 to +13 and shorts from -8 to +21

This data was logged over a 45 minute drive 40% city 60% highway.

I have around 215000km on the truck O2's are probably original. The engine has about 80,000km on it.

It gets a shudder at idle that me or my mechanic can't pinpoint. OBD shows no misfires.

Any suggestions on where to look, or just replace the cat and see how she goes?

 

@Bonder, logging o2 sensors every second robs you of much of the benefit of logging. The PCM reads voltage of upstream O2's about six times a second and can switch direction of fuel trim _up to_ every read cycle. Two to three switches per second wouldn't be uncommon. Your log readings indicate that the O2 sensors do at least work - but you're seeing readings grabbed at 1 second intervals which doesn't tell much more than that.


However - whatever their readings are at this point, (based solely on their reported 'likely' mileage) I would recommend replacing all four no matter what else you do. You'll receive benefit even in conjunction with whatever else you do. The ENTIRE closed loop fuel delivery system is totally dependent on good, clean, calibrated voltage readings from them. Just because their voltage ranges from .1 to .9 (normal), the center point (.45v) should represent the sensing presence of oxygen representing proper fuel/air ratio. Just consider - what if that calibration point has drifted to .3v or .7v, --- throws the whole thing out of whack.


O2 sensors _DO_ become 'lazy' with time, heat cycles and getting coated with exhaust soot - etc. I recommend replacing them as preventative measure every 100,000 miles.


As for your P0420 code, in my opinion, it could be completely the result of "LAZY" O2 sensors. The PCM runs a monitor routine every drive cycle checking Catalyst efficiency. It involves (among other things) 'intentionally richening' fuel trim (driving them "+") for a time swamping the catalyst, then intentionally 'leaning' fuel trim (driving them "-") and verifying the catalyst is doing its job properly by reading voltage from rear O2. The sensors becoming 'LAZY' upsets the PCMs ability to make a good judgment on that test - and could result in the P0420 (and soon a P0430).

The above 'calibration' issue could be the cause of the 'generally' elevated fuel trims. They appear to be running more above Zero than they do below zero, or somewhat high (too much fuel / poor mileage). Hasn't reached P0171 or P0174 point yet, but its near (LTFT +25 & STFT +5 for > 2 seconds) This is placing heaver load on the catalyst and can result in (overheating) damaging them. I would replace O2s first, then attack the elevated fuel trims by first making sure there is no air getting into the intake that is not metered by the MAF. ie: vacuum leak common to both banks.

 

OK thanks for the input. I ordered O2's today, so now to see if I can even attempt to remove the passenger side one.

I was going to do the fuel filter as well, and replace the plugs. (Had that planned before it threw the code)

Only place I can think of it drawing air from might be the filter tray where some foam is missing. I also know my intake is missing a part. On the breather tube it just stops after the corrugated portion, so it is just drawing air from under the hood. Not sure why the previous owner did that, unless it gave better mileage at -40.

 

Yeah, that passenger side one is a B*@$)!. I've done mine twice in 228k miles. I removed the RF wheel and fender liner. Had a helper reaching over the frame - between firewall and bell housing - to put position the O2 socket while I ratcheted it from underneath.


You need a helper with small hands instead of 'Kiolbassa' sausage fingers like mine to get the wire connector unplugged and re-plugged. But if this 71 year old can do it, I have confidence that YOU can.


I just noticed where you are from. Guess you guys are more interested in Hot Air Intakes than Cold Air Intakes like us cowboys from these parts. For a good laugh at my expense on that subject, take a look at this post: https://www.f150forum.com/f4/definit...3/#post5470333

 

LOL that is funny. The plan was to start this weekend on it, but it seems monsoon season has rolled in.

There was enough hail on the ground Friday night to make it look like snow. Supposed to be nicer this weekend