P0171 Dtc

[coffey2]

I realize questions about the DTC P0171 are asked a lot, but I have not seen an answer to this particular question.

My Buick is an 02 LeSabre Custom. I believe it has one 02 sensor before the CAT and one after the CAT. Therefore, I think the P0171 is not specific to the left bank, but rather to the whole engine. It is the equivalent to getting both P0171 and P0174 DTCs for engines with three or four 02 sensors. I ask this, because for the longest time I have been chasing after a malfunction affecting only the left bank, when it might be something affecting both banks, like low pressure, MAF or something else that affects both banks.

All I need is confirmation that P0171 for this engine may apply to both banks.
Thanks

[billn2]

Your'e right it has only 2 02 sensors, pre-cat and post-cat. Code P0171 means your engine is running lean. I don't see that on general motors vehicles very often, but I do see a lot of Fords with that code. 99% of the time it is a vaccum leak, might be as simple as a vaccum hose came loose or maybe even the intake manifold gasket could be leaking. It's hard to diagnose a car over the internet, but on your Buick I'd guess you've got a vaccum hose off somewhere.

[BNaylor]

Originally Posted by coffey2

All I need is confirmation that P0171 for this engine may apply to both banks.
Thanks

Welcome to AF.

To start there is no Bank 2 so to answer your question the P0171 DTC which is not uncommon on a SII 3800 engine applies as a valid DTC.

As mentioned DTC P0171 indicates the system is in a lean trim mode. Ideal fuel trim = zero. Greater than zero indicates the PCM is adding fuel to compensate for the lean condition. If fuel trim is less than zero the PCM is reducing the amount of fuel to compensate for a rich condition.

There are many possible causes for this specific DTC and it may not be easy to pinpoint the exact cause if it is not obvious and further diagnostics or parts swapping is needed. With a full function odb-ii scan tool you can check short and long term fuel trim values and that may help pinpoint the cause better.

Here are some possible causes.

Vacuum leak . Check all vacuum hoses at intake system. Possible supercharger or LIM gaskets.
PCV Valve
Possible throttle body blockage (clean input screen and body)
IAC (idle air controller valve)
Fuel injectors (bad lower o-ring seal)
MAF sensor
EGR valve
Bank 1, Sensor 1 02 sensor (Pre-CAT converter)

[coffey2]

I want to thank the two responses I got. It kept me looking for a vacuum leak which I found. (However, I bought a fair amount of diagnostic equipment that I hope will be useful in the future.) Anyway it was a valve in the back of the engine that provides vacuum to the brake booster canister. It had broken a hose and was sucking air big time. After capping it, the engine LTFT returned to around +5.

Again thanks.

[bob60506]

I had the P0171 error code on our 2005 Buick LeSabre and it turned out to be a vacuum leak. Thanks to those who provided suggestions here, I had a very good idea of where to check right away -- and it turned out to be as I suspected.

You see, about a year ago, I replaced the thermostat -- and accidentally broke a nearby vacuum line. Just to keep the car running until I could get a replacement line, I managed to seal it with glue and electrical tape.

I got some replacement tubing right away, but my glue/tape job was holding up so well that I decided to let it ride. Finally, I got the "Service Engine Soon" light a few days ago. The car seemed to be running fine, so I suspected a faulty O2 sensor. Rather than spend the money on one right off the bat, I decided to check online for troubleshooting help. When I found this thread, I pulled off the now-loose tape and removed the old glued line (which came apart fairly easily) and put in that replacement tubing that I had been holding onto all this time. Voila! Problem fixed.

Thanks, guys!