F150 with codes 0172 and 0175

[harriw]

Hey folks,

I know this might below in the full-size forum, but I'd like to ask those knowledgable with Ford engine computers in general, not just specifically F150's (though i don't really know if they work in the same way or not). If I'm out of line, I appologize - please feel free to switch this over to the full-size forum if you think it'd be more appropriate there.

Anyway.... I've got an '03 F150 4x4 lariat with the 5.4. CEL is on with codes 0172 and 0175 - both banks running rich.

My cousin brought his good scanner over, and we had a look at the realtime data. Sure enough, long-term fuel trim is pegged low (around -20%), meaning the computer is trying to lean the mix out as hard as it can but can't get there. So we took a look at the O2 sensor readings, and here's where it gets interesting. F150's have 2 cats and 4 sensors - one before and one after each cat. The upstream O2 sensors on both sides look like you'd expect them to be normally - they're bouncing from around 0.1V to 0.7V, as if they're jumping from lean to rich like they're supposed to. The downstream sensors are both sitting at 0.7V though (rich).

So, any thoughts on what this indicates? I guess I'm not sure which set of sensors the computer is using to trim the fuel mix. I would think it would use the upstream sensors, but they seem to be working normally so why the codes? Unless both upstream sensors are off together, and the downstream ones are correctly showing rich? But the cats should have burned off all that extra fuel right?

What do you guys think? Does this look like bad upstream sensors, bad cats, or bad downstream sensors?

Of course the other thing is that any of those 3 options mean that 2 sensors or 2 cats went out together, which would be unusual... Is there any single failure that might produce the data seen here?

Just some background, the guy I bought the truck from said the CEL was for the MAF sensor (he didn't say which code it was). Said he cleaned it, but it didn't go away. First thing I did was clean the MAF again, then pulled the neg battery cable to clear the code and see if it would come back. In hindsight, I should have had the code read first at least so I knew what was in there originally. Anyway, after cleaning the MAF and seeing the rich codes show up, I checked the MFA's output voltage. It's spot-on where it ought to be for various engine RPM. Air filter looks pretty good as well.

Anyway, any input would be greatly appreciated. Just thought I'd run it past you folks before I pay a dealer to have a look at it. Thanks a lot!

-Bill

 

[alwaysFlOoReD]

Is it possible to swap the up and down stream sensors, so the downs would be in the upstream side?

Richard

 

[harriw]

Hm... hadn't thought of that. They're listed as different parts (with different prices), but I believe it's only because the wiring harness is longer one one than the other. Not sure the wires would reach on one of them (or even if the plugs are the same...)

I'll definitely look into that though - thanks!

 

[adsm08]

Ignore the downstream sensors.

Keep in mind, the o2 sensor is a crystal that produces voltage in the absence of oxygen. Low voltage means high oxygen content and a lean condition (low and lean). Higher voltage indicates all the oxygen is being burned and that there is likely fuel being dumped down the exhaust. On the upstream sensor readings.

The converter's job is to remove nitrogen from the exhaust and to combine the CO2 and O2 into water. If the converters are working properly the downstream sensors should read .6 to .8 V normally as there will be little detectable oxygen in the exhaust after the converters.

The downstream sensors are used to determine if the cats are working and that is their ONLY purpose in life. You can swap them, but it sounds like they are working fine and that you have another issue.

Did you look at the MAF signals? MAP, IAT, BARO? Do they all line up. Lean condition is usually an air leak. Rich is a stuck injector (usually only affects 1 bank) or a MAF issue.

 

[harriw]

Thanks adsm08 - that's all great info, and exactly the stuff I'm looking for

So am I correct in assuming then that it's the upstream O2's that the PCM uses to tweak fuel mix? I'm confused then why if the upstream O2's look normal that this code is popping at all? Even if it's caused by something else, I'd expect to see the upstream O2's reading high (rich) consistantly?

Anyway, interesting you mentioned MAF. According to the guy I bought it from, the original codes did indicate the MAF. And I do see some residue on the sensor. But as I said, I checked its voltage output and it looked correct: about 1V at idle, and it went up with engine RPM. I don't recall the exact numbers, but it matched some target numbers I found online somewhere or other.... I'll look into the other things you mentioned.

Thanks a lot!

 

[harriw]

Oh, and as you aluded to, the fact that I've got codes for BOTH banks makes me think it's unlikely (though still possible) it's something like an injector (or even O2 sensor) that would affect only one. i.e. what are the odds that something goes wrong in BOTH banks at the SAME time that causes the same code.... (of course it's possible, I'm just saying unlikely, making me think it's likely to be something that affects both banks).

Thanks again!

 

[adsm08]

I believe there is a TSB for many vehicles that says if you have two rich codes to clean the MAF (use real MAF cleaner or you will destroy the sensor) and clear codes and recheck it.

That's what I'd do.

And yes, it's only the upstream sensors that figure into fuel mix.