Intermittent P0171 code causes

Hoping for some help from the electrical sleuths here - 2010 2TR-FE throws a CEL, reader shows P0171 code. Live data shows MAF functioning and voltage on upstream sensor shows normal voltage (3.2-3.4). Truck runs fine, no roughness, no power loss, no leaks, mpg is unaffected. After about 2 weeks, code disappears (no CEL, no code). That was 3 weeks ago, hasn’t come back on again.

What would cause an intermittent code like this?

 

Have you cleaned your maf? Sometimes that helps or it could have just been a loose gas cap if the code stays off. I’d wait to see if the code comes back on before cleaning the maf if you haven’t already since sometimes cleaning the maf can make it worse.

 

We would need to look at fuel trims and AFR/O2 voltages at various operating conditions (idling cold, idling hot, acceleration, cruise, & WOT) to give direction on a lean condition.

+/- 10% Total (Long Term + Short Term) trim is the normal range, more than +/- 15% I would consider an issue that should be addressed, and anything more than +/- 35% will set a lean/rich code. More than +/- 50% will be outside of fuel control range and would likely cause the engine to run poorly.

Generally with a vacuum leak you would see high positive trims at idle and they would improve with an increase in RPM/load. Gasket leaks (such as intake gaskets) tend to be worse the colder the engine is and improve as the engine heats up.

Generally with a dirty MAF sensor you would see normal to negative trims at idle and trims would go positive with increasing RPM/load.

Generally a fueling issue (low fuel pressure or volume) depending on how weak the pump is you would see normal to positive trims at idle and they would increase with load.

 

Thanks for the input guys. So the code popped up again (of course, the day after I made the first post). I have a feeling it’s the upstream O2 sensor as the downstream one did the same thing when it went bad (coded, disappeared for a time, then reappeared).

So to further test this before buying a new sensor, I would just check the live data voltages on the O2 sensor for the % variance under the different throttle conditions specified, correct?

 

Don't often see an AFR/O2 sensor cause rich/lean codes without setting an AFR/O2 sensor code.

Have you done the checks I listed above to see what kind of lean condition we are dealing with?

 

I have not; admittedly I’m not sure I know how. I thought it might be something I could look at via live data on the code reader but I can’t seem to find it.

 

You should be able to see those parameters if your scan tool can view live data.

 

I'd be curious what freeze frame data shows.
Only because you said it came on, then went off on it's own for 3 weeks.
And now it's back.

Freeze frame won't tell the whole story. But it might give us a clue to where to start looking.

But we really need to know the fuel trims currently, no matter what.

 

To update, the code disappeared again for a couple of days and then reappeared. Also noticed the mpg is a couple miles less than normal. I was able to look at the short and long term trims and the ST is in the +/- 10 range, but the long trim shows 28% at idle and drops lower under load. This would indicate a vacuum leak somewhere, yes? Any common places I should have a look? I’m not smelling any fuel under the hood.