P0171 False Lean

[Firstborn10]

So for the last few months I've been getting p0171(bank 1 lean) on and off on my 02 (50 state emissions). I have a scangauge, so I punched in the Xgauge codes for Bank 1 short and long term fuel trim, and sure enough both are high, with long term getting up as high as mid 30's. Thing is.... The exhaust smells STRONGLY of gas. So it's obviously not running lean like it says, but in fact is running rich.

Naturally I cleaned MAF and checked vacuum lines, so my next guess would be upstream O2 sensor. Only problem is I replaced both O2 sensors with new Denso sensors a couple years ago.

My question is A) is it possible for a sensor to go bad that fast? And B) if not the upstream O2 sensor then what could it be?

 

[Bassochist]

After cleaning my MAF my issue went away for a week or two then started to come back.
And New MAF ($100 cad) fixed this up.
Maybe the MAF is just done for?

 

[Bassochist]

And right after clearing my 0171 lean I got an 0125 for rich. Haha upstream 02 swap cleared that.

 

[gamefreakgc]

So for the last few months I've been getting p0171(bank 1 lean) on and off on my 02 (50 state emissions). I have a scangauge, so I punched in the Xgauge codes for Bank 1 short and long term fuel trim, and sure enough both are high, with long term getting up as high as mid 30's. Thing is.... The exhaust smells STRONGLY of gas. So it's obviously not running lean like it says, but in fact is running rich.

Naturally I cleaned MAF and checked vacuum lines, so my next guess would be upstream O2 sensor. Only problem is I replaced both O2 sensors with new Denso sensors a couple years ago.

My question is A) is it possible for a sensor to go bad that fast? And B) if not the upstream O2 sensor then what could it be?

It sounds like a massive vacuum leak. Check also your exhaust for a leak or even around the O2 sensor itself. If it leaks at the mount it will give some bad readings.

The lean code doesn't mean the engine is running lean, it means it cannot compensate enough to get the air to fuel ratio back to stoich of 14.6 and is reading something higher than that. Most common issue is unmetered air entering the intake or exhaust and messing up the reading. All it takes is a leak after the MAF (which measures how much air is coming in) and before the front O2 sensor (which measures how much air is coming out) and it throws the whole system for a loop, since it's not adding enough fuel for the amount of air the sensors are reading. Exhaust leaks mess it up the most since outside air can mix right in with the exhaust and then the sensor sees a ton of oxygen vs what is really happening.

A couple of years is too soon for a sensor to normally go bad. They last roughly 100k miles, sometimes longer.

 

[Firstborn10]

Thank you guys for your input. I had not considered an exhaust leak! I took it to a mechanic yesterday for a diagnosis and they said they found a vacuum leak and sealed it. It shut off the code, however my fuel trims are still all out of wack so that clearly wasn't it. Ill check out my exhaust and report back!

 

[TheDurk]

Don't neglect a good throttle body cleaning, if it hasn't been done.