P0171 Code on a Buick: Fuel Trim System Lean, Bank 1 (Buick Guide)

P0171 on a Buick is "Fuel Trim System Lean, Bank 1." The engine computer has been adding fuel to bank 1 for so long that it has reached its correction limit — something is letting in air the engine did not measure, or the engine is not getting the fuel it asked for. On a Buick the first question is not "which sensor?" but "which engine?", because GM's own documents point to very different culprits: a torn PCV diaphragm on the 1.4L turbo Encore, a leaking foul-air tube on the 3.6L Enclave, loose air-duct clamps on the Verano Turbo, and a purge valve that will not close on several 2016–2020 Buicks. Two of those purge-valve problems are covered by GM special coverages, and one of them still has years left to run.

What Does P0171 Mean on a Buick?

Your Buick's engine control module (ECM) uses the upstream oxygen sensor to trim fuel delivery many times a second. Quick corrections are short-term fuel trim; the correction it learns and carries forward is long-term fuel trim. Positive numbers mean the ECM is adding fuel because it sees a lean mixture.

P0171 sets when bank 1's long-term trim stays at its lean-correction limit. As one example, GM's OBD documentation for the 2008 Enclave's 3.6L V6 (RPO LY7) sets P0171 when the weighted average of long-term fuel trim reaches a factor of 1.23 — roughly 23% more fuel than base — in closed loop, with no misfire, oxygen sensor, EVAP, injector, MAF, MAP or throttle codes present. It is a Type B code, so it normally needs two consecutive failing trips before the check engine light comes on. Thresholds differ by engine and model year, but the logic is the same across the Buick lineup.

P0171 is a conclusion, not a part: find the air leak or fuel shortfall behind it.

For the general GM background, see our P0171 GM diagnosis guide. This page focuses on what GM has documented for Buick models.

Which Buicks Set P0171

Every Buick with an oxygen sensor can set P0171. What changes is what "bank 1" means and whether a second lean code exists.

• Four-cylinder and three-cylinder Buicks — Encore 1.4T (LUJ/LUV/LE2), Verano 2.4L and 2.0T, LaCrosse and Regal 2.4L (LAF/LEA), Regal and Envision 2.0T (LHU/LTG), Envision 2.5L (LCV), Encore GX and Envista: there is only one bank, so P0171 covers the whole engine. There is no P0174 on these engines.
• 3.6L V6 Buicks — Enclave, LaCrosse V6 and Regal GS V6: bank 1 is the bank containing cylinder 1. On these transverse V6s that is the rear bank, nearest the firewall. Bank 2 has its own lean code, P0174.
• 3800 V6 Buicks (LaCrosse, Allure and Lucerne 3.8L): GM's 2008 OBD documentation for the 3.8L (L26) lists P0171 and P0172 but no bank 2 fuel trim codes, so on these engines P0171 covers both cylinder banks.

The pairing is the first diagnostic fork on a V6. P0171 and P0174 together point at something both banks share — the MAF sensor, a purge valve stuck open, a PCV or intake leak, fuel pressure. P0171 alone on a 3.6L points at something affecting the rear bank only, such as a leak at that bank's intake runners or an injector issue on that side.

The Purge Valve Coverages

This is the Buick-specific detail most P0171 pages miss. GM released special coverage N192210240 in September 2019 because a condition on certain vehicles could cause the evaporative emissions purge valve not to fully close. A purge valve stuck partly open lets fuel vapor and unmetered air into the intake, which can make the engine run rough, hesitate or stall at idle and set a code. GM's service procedure names P0496, P0442, P0455, P0171 and P0174 as the codes these vehicles may arrive with.

The Buicks covered by N192210240 are:

• 2016–2017 Buick Encore
• 2016–2017 Buick Enclave
• 2016 Buick LaCrosse
• 2018 Buick Cascada

Coverage runs 10 years or 150,000 miles for Enclave and Cascada, and 10 years or 120,000 miles for Encore and LaCrosse, whichever comes first, from the date the vehicle was originally placed in service. Many 2016 vehicles have now aged out. Some 2017 Encores and Enclaves and 2018 Cascadas may still be inside the term, depending on in-service date and mileage. The reimbursement deadline for repairs paid before the coverage (September 30, 2020) has passed.

GM followed up in June 2023 with special coverage N232395300 for the same purge valve condition on the 2019 Buick Enclave with the 3.6L (LFY) and the 2020 Buick Encore with the 1.4L turbo (LUV). This one runs 15 years or 150,000 miles, so for practical purposes it is still active on most of those vehicles. The service procedure lists the same five codes, P0171 included.

Two details from these documents are worth knowing before you visit a dealer:

• The coverage is diagnosis-led. If following GM's service information for the code does not lead to the purge valve, the dealer claims diagnosis time and anything else is customer pay.
• GM lists more than one purge valve part number and tells dealers to order by VIN. Do not assume the valve on a shelf fits your Buick.

What GM Found Engine by Engine

Encore 1.4L turbo (LUJ, LUV, LE2)

GM bulletin 19-NA-114 covers the 2013–2019 Encore and the related 1.4L Chevrolets. The cause GM describes is a torn PCV crankcase pressure control diaphragm in the PCV valve, which lets unmetered air into the crankcase. GM's key instruction: confirm the PCV valve's performance before replacing the camshaft cover, because the valve is integrated into it. The bulletin uses a simple tissue test at the PCV vent with the engine running — a torn diaphragm holds the tissue against the port.

The earlier preliminary information PIP5197B covered 2013–2014 Encores with oil consumption, oil leaks, blue smoke or fuel trim codes, with P0171 among the listed DTCs.

A separate preliminary information, PIP5240F, covers 2013–2019 Encores with P0171 and/or P1101: if normal diagnosis finds nothing, check that the air cleaner housing has the correct internal content for the engine. GM notes the LE2 has both a hydrocarbon adsorber and a diffuser, while the LUV has only the adsorber.

Enclave 3.6L V6 (LLT, LFY)

GM's PIP5549A covers 2012–2019 Enclaves with P0171 and/or P0174. GM's suspect is a vacuum leak at the foul-air (PCV) tube or the purge path. The test: record trims hot, disconnect and plug the foul-air tube and its manifold port, then watch the trims. If positive trims swing negative, replace the tube. GM notes the intake manifold may sometimes need replacing too, but only if the test changed the trims. If not, check whether the purge valve passes vacuum with its hose and connector removed.

LaCrosse and Regal 2.4L (LAF), Verano 2.0T

PIP5028D covers 2010–2011 LaCrosse and Regal 2.4L cars with P0128 and/or P0171 and no driveability complaint. GM's order: normal diagnosis first, then an ECM calibration update, then a checklist — oil leaks drawing air through the PCV system, a dipstick or oil fill cap not seated after an oil change, fuel ethanol content above 15%, a smoke test, a non-factory or damaged air filter, and casting flash in the air cleaner before the MAF. If long-term trims are in the mid-teens at hot idle, GM suggests plugging vacuum lines one at a time and watching for trims to fall to 10% or less.

For the 2013 Verano with the 2.0L turbo, PI0931 is refreshingly simple: P0171, P1101 and/or P0101 can come from loose clamps on the air cleaner outlet duct. Tighten them.

Regal and Envision 2.0T (LTG) and 2.5L (LCV)

PIP5529B covers the 2014–2017 Regal and 2016–2017 Envision with a list of codes including P0171, plus hesitation or stall. If service information does not find the cause, GM says to check the intake manifold bolts for being loose, re-torque them with threadlocker, and not to replace the gasket unless it still leaks afterward.

Symptoms of P0171

• Check engine light on, often with no driveability complaint
• Rough or hunting idle, especially when warm
• Hesitation or stumble on light throttle
• Stalling at idle (a classic sign of a purge valve stuck open)
• P0174 alongside on a 3.6L V6 when the cause is shared
• EVAP codes alongside (P0496, P0442, P0455) when purge is involved
• P1101 or P0101 alongside on turbo Buicks when the leak is in the intake ducting

Causes and How to Confirm Each

How Is P0171 Diagnosed?

1. Record every code and the freeze frame. Fix misfire, MAF, MAP, injector and O2 circuit codes first — GM blocks the fuel trim test while those are set.
2. Check whether P0174 is also set on a V6. Both banks lean means a shared cause.
3. Read live fuel trims hot at idle and at a steady 2,500 rpm. Lean at idle that improves with rpm points at a vacuum leak; lean under load points at fuel delivery or MAF.
4. Look for the easy wins: dipstick and oil cap seated, air duct clamps tight, filter correct and undamaged.
5. Test the purge valve for vacuum at the port with its hose and connector removed, or command purge on and off and watch the trims.
6. Test the PCV system the GM way for your engine: tissue test on the Encore 1.4T, foul-air tube plug test on the Enclave 3.6L.
7. Smoke test the intake to find leaks you can't hear.
8. Clear codes and drive long enough for closed-loop operation so the fuel trim monitor can run again.

Parts and Cost

GM Documents That Name P0171 on Buick Models

Have a dealer check your VIN against current bulletins and special coverages rather than relying on any article, including this one. The check is free.

What Other P0171 Pages Get Wrong

• "Replace the O2 sensor." An oxygen sensor reporting lean is usually reporting the truth. None of the Buick documents above fixes P0171 with an O2 sensor.
• Ignoring the purge valve. GM wrote two special coverages that list P0171 among the purge valve codes on Buicks. A stuck-open purge valve is a vacuum leak with a part number.
• Replacing the Encore's camshaft cover on sight. GM's 19-NA-114 says to confirm the PCV valve is actually faulty first.
• Replacing the Enclave's intake manifold first. PIP5549A says the foul-air tube comes first, and the manifold only if testing proves the leak.
• Promising the purge coverage will pay. It is diagnosis-led and time-limited, and many 2016 vehicles have aged out.

Frequently Asked Questions

Is P0171 serious on a Buick?

Usually not an emergency, but a lean engine can stumble, stall at idle and run hotter in the combustion chamber. If the cause is a purge valve stuck open, stalling at a stop is a real nuisance. Diagnose it soon rather than clearing it and hoping.

Is my Buick covered by the purge valve special coverage?

N192210240 lists the 2016–2017 Encore and Enclave, 2016 LaCrosse and 2018 Cascada; N232395300 lists the 2019 Enclave 3.6L (LFY) and 2020 Encore 1.4L (LUV). Coverage depends on your VIN, in-service date and mileage, and the dealer must find the purge valve to be the cause. A dealer VIN check is free.

Which side is bank 1 on a Buick Enclave?

Bank 1 is the bank with cylinder 1. On the transverse 3.6L V6 in the Enclave, LaCrosse and Regal, that is the rear bank, nearest the firewall.

Why does my Encore have P0171 and P1101 together?

Both point at unmeasured air. GM's documents for the Encore send technicians to the PCV diaphragm (19-NA-114), the air cleaner housing content (PIP5240F) and intake leaks generally.

Can a dirty MAF sensor cause P0171 on a Buick?

Yes, an under-reading MAF makes the ECM under-fuel. Test it against service information values before replacing it, and check for duct flash, an incorrect filter or a loose clamp first, as GM's PIP5028D and PI0931 suggest.

Can I drive with P0171?

Short term, usually, if it runs smoothly. If it stalls, misfires or the light flashes, have it checked first.