P0172 Code on a Buick: Fuel Trim System Rich, Bank 1 (Buick Guide)

P0172 on a Buick is "Fuel Trim System Rich, Bank 1." The engine computer has been pulling fuel out for so long that it has reached its limit, and the mixture is still rich. On the Buick most likely to set it — the Regal or Envision with the 2.0L turbo (LTG) — GM has said in two bulletins that the usual cause is not a bad part at all. It is gasoline collecting in the engine oil from short trips, and GM states that replacing the high-pressure fuel pump or injectors will not fix it unless those parts are proven to be causing a rich condition. The repair GM specifies starts with an oil change.

What Does P0172 Mean on a Buick?

The engine control module (ECM) in your Buick trims fuel using the upstream oxygen sensor. Short-term fuel trim reacts instantly; long-term fuel trim is the correction the ECM learns and keeps. Negative trim means the ECM is removing fuel because the exhaust looks rich.

P0172 sets when bank 1's long-term trim stays at its rich-correction limit. GM's rich test has a twist that matters for diagnosis. In GM's 2008 OBD documentation for the Enclave's 3.6L V6 (LY7), for example, the rich test is passive and, if needed, intrusive: when trims look rich, the ECM ramps off EVAP purge and re-checks. Only if the mixture is still rich with purge off does it fail. It is a Type B code, so the check engine light normally needs two failing trips.

That intrusive step tells you something useful: a P0172 that survives it is not just fuel vapor from the charcoal canister. Something else is adding fuel — and on a direct-injected Buick turbo, one of the most common "something elses" is fuel boiling out of the oil and being drawn back in by the PCV system.

For a Chevrolet four-cylinder take on the same code, see our P0172 guide for the Chevy Equinox.

Which Buicks Set P0172

• Four-cylinder Buicks — Regal, Envision, Verano, LaCrosse, Encore and others: bank 1 is the whole engine. There is no P0175 on a four-cylinder.
• 3.6L V6 Buicks — Enclave, LaCrosse V6, Regal GS V6: P0172 is bank 1, the bank with cylinder 1. On these transverse V6s that is the rear bank. The bank 2 rich code is P0175.
• 3800 V6 Buicks — LaCrosse, Allure and Lucerne 3.8L: GM's 2008 OBD documentation for the 3.8L (L26) lists P0172 but no bank 2 fuel trim code, so P0172 covers the whole engine.

The Buicks GM has named in P0172 documents are the Regal and Envision with the 2.0L turbo (LTG), and the LaCrosse, Regal and Verano four-cylinders in a 2026 preliminary information about aftermarket high-pressure fuel pumps. A 2010 LaCrosse also appears in an earlier GM preliminary information about P0172 and P0175 together.

The Fuel-in-Oil Bulletins

GM bulletin 18-NA-246 (first released February 2019, revised March 2022) covers the 2014–2019 Buick Regal and 2016–2019 Buick Envision with the 2.0L turbo (LTG), along with LTG-equipped Cadillacs, Chevrolets and GMCs. GM's description of the cause:

"This condition may be caused by driving the vehicle in short intervals, allowing excessive fuel to collect in the crankcase."

The revised bulletin adds that DTCs P06DE and/or P0521 (oil pressure related) may also be set, and limits the affected range by build date and engine unique number. GM's procedure depends on model year: an ECM software update on earlier vehicles, a revised primary PCV valve on 2016–2019 vehicles (on the Envision this means removing the camshaft cover), and an oil and filter change on every vehicle to get the fuel out of the crankcase.

For the 2020 Buick Regal and Envision, GM issued a companion bulletin, 22-NA-056. It is the clearest description of the test:

• If P0172 is caused by fuel in the crankcase, long-term fuel trim should be a large negative number — GM gives −18% to −30%.
• Unplug the MAF sensor. If long-term trim does not move into a normal range of −10% to +10%, check the oil for being overfilled and smelling of fuel.
• If fuel dilution from short trips is confirmed, change the oil and filter, clear the code and road test.

Both bulletins carry the same warning: replacing fuel components such as the high-pressure fuel pump or fuel injectors will not correct P0172 under these conditions unless those parts are verified as causing a rich condition. And both say that if the owner keeps driving this way, the oil must be changed more often than the oil life monitor indicates.

So on a turbo Regal or Envision with P0172, the first tool is the dipstick. Oil above the full mark that smells of gasoline is your answer — and a lot cheaper than a fuel pump.

High-Pressure Pump Cases

A fuel pump can still be the cause. GM's PIP6113 (May 2026) covers stalling at a stop with P0191 and occasionally P0172 on the 2010–2016 LaCrosse 2.4L, 2010–2017 Regal 2.0T and 2.4L, and 2012–2017 Verano 2.0T and 2.4L. GM's cause: an aftermarket high-pressure fuel pump failure. The check is to look at the markings on the pump body — an original GM pump carries a QR code and a GM part number. If it is not a GM pump, GM says to install one and re-evaluate.

That is worth checking on any of these Buicks if the pump has ever been replaced. It is also a reminder that "replace the pump" can create the problem as well as fix it.

Earlier, GM's preliminary information PI-0145 described a check engine light with P0172 and P0175 and rough running on the 2010 LaCrosse (and related Cadillac, Chevrolet and GMC models), with a high-pressure fuel pump replacement after diagnosis. We have seen only NHTSA's summary of that document, not the document itself.

One bulletin you may see cited for P0172, 20-NA-098 (fuel injector flow classes), applies to Cadillac, Chevrolet and GMC V6 and V8 engines. It does not list Buick.

Symptoms of P0172

• Check engine light, often with no driveability complaint
• Oil level above full and smelling of gasoline
• P06DE or P0521 alongside on the 2.0T (per 18-NA-246 and 22-NA-056)
• Fuel smell from the exhaust or oil fill
• Poorer fuel economy
• Rough idle or stall at a stop (high-pressure pump cases, with P0191)
• Black soot at the tailpipe on a severe rich condition

Causes and How to Confirm Each

How Is P0172 Diagnosed?

1. Record all codes and freeze frame. Repair misfire, MAF, MAP, injector, O2 circuit and EVAP codes first — they block the fuel trim test.
2. Pull the dipstick. Overfull and smelling of fuel is the strongest clue on a turbo Buick.
3. Ask how the car is driven. Frequent short trips that never fully warm the engine fit GM's bulletins exactly.
4. Read long-term fuel trim hot. GM's 22-NA-056 expects −18% to −30% with crankcase fuel.
5. Unplug the MAF and watch trims, per 22-NA-056.
6. Command purge off and see whether the trims recover.
7. Check fuel pressure desired vs actual, and identify the high-pressure pump on LaCrosse, Regal and Verano four-cylinders (PIP6113).
8. Have a dealer check the VIN for 18-NA-246 or 22-NA-056 applicability, software level and PCV valve revision.
9. Change the oil and filter when fuel dilution is confirmed, clear the code and road test.
10. Adjust oil change intervals if the driving pattern will not change — GM's own instruction.

Parts and Cost

GM Documents That Name P0172 on Buick Models

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

What Other P0172 Pages Get Wrong

• "Replace the fuel injectors or fuel pump." On the Regal and Envision 2.0T, GM says those parts will not fix P0172 caused by short-trip fuel dilution unless they are proven faulty.
• Skipping the dipstick. The cheapest test on a turbo Buick is also GM's first one.
• Blaming the O2 sensor. A sensor reporting rich on a fuel-diluted engine is reporting the truth.
• Treating a new fuel pump as safe. GM's PIP6113 traces stalling and P0172 to aftermarket high-pressure pumps on Buick four-cylinders.
• Citing 20-NA-098 for a Buick. That injector bulletin lists Cadillac, Chevrolet and GMC only.
• Assuming the oil life monitor protects you. GM tells owners who keep making short trips to change oil more often than the monitor says.

Frequently Asked Questions

Why does my Buick Regal have P0172 and oil that smells like gas?

Short trips let fuel collect in the crankcase, and the PCV system feeds that vapor back into the engine, making it run rich. GM's 18-NA-246 and 22-NA-056 describe exactly this on the Regal and Envision 2.0L turbo.

Will an oil change fix P0172?

If fuel dilution is confirmed, an oil and filter change is GM's correction, sometimes with a software update and revised PCV valve on 2014–2019 models. If trims stay rich afterward, keep diagnosing.

Is P0172 serious?

Not usually an emergency, but fuel-diluted oil protects the engine less well and a sustained rich mixture is hard on the catalytic converter. Fix it rather than living with it.

Can a bad O2 sensor cause P0172 on a Buick?

A failed sensor normally sets its own codes. None of the Buick documents we found fixes P0172 with an oxygen sensor.

Can my Buick Enclave set P0172 and P0175?

Yes. The 3.6L V6 has two banks; P0172 is bank 1 and P0175 is bank 2. Both together point at a shared cause such as fuel pressure, purge or fuel in the oil.

Does the purge valve special coverage cover P0172?

No. GM's purge valve coverages N192210240 and N232395300 list lean codes (P0171, P0174) and EVAP codes, not P0172.

How do I stop P0172 coming back?

If short trips are the cause, combine errands so the engine fully warms up, and change oil more often than the oil life monitor indicates, as GM advises.