The P0171 code means the engine computer on bank 1 has been adding more and more fuel and has hit its limit. Its official name is “system too lean bank 1”. The engine is getting more air than the computer accounted for, or less fuel than it asked for, and in most cases the extra air is sneaking in somewhere it should not.
How P0171 sets: the computer chases missing fuel
The mass airflow (MAF) sensor measures the air coming in, and the computer injects fuel to match. The oxygen sensor in the exhaust then checks the result. If it keeps reading lean, the computer raises a correction called fuel trim. Once that correction stays too high for too long, it sets P0171.
Air that enters after the MAF sensor is the usual reason. The sensor never saw it, so the computer never added fuel for it. A cracked duct, a split hose or a failed PCV valve lets air in this way.
When P0171 and P0174 appear together
P0174 is the same fault on bank 2. When both codes are stored, both sides of the engine are lean, which points to a cause they share, such as the MAF sensor or fuel pressure. A leak at a single intake runner or one injector usually shows up on one bank only. Most inline four-cylinder engines have only bank 1, so they can only set P0171.
What carmakers’ bulletins point to
NHTSA holds 247 manufacturer bulletins that mention P0171. Counting each revision once and keeping repair bulletins for US vehicles leaves 149.
What manufacturers prescribe for P0171
Share of 149 distinct repair bulletins that mention P0171, by the fix they call for
Source: NHTSA manufacturer communications (technical service bulletins), received 2010–2026. Revisions of the same bulletin are counted once. A bulletin can call for more than one fix.
Engine software updates lead again, most of them for Stellantis brands such as Jeep and Ram. Behind that come the PCV system and other air leaks, along with fuel delivery problems. Several of the GM air leak bulletins are special coverage programs, which means the repair is free for vehicles that still qualify.
| Bulletin | Vehicles | What it says |
|---|---|---|
| N202299080 Chevrolet · Jul 2020 | Cruze 2011–2014 Sonic 2012–2014 Trax 2014 | The PCV valve built into the camshaft cover of the 1.4L turbo can crack and let extra air into the intake. Covered under special coverage. |
| N192210230 Chevrolet · Jul 2019 | Cruze 2015–2016 Sonic 2015 | The same cracked PCV diaphragm on later 1.4L Cruze and Sonic models, also under special coverage. |
| GCUS-9-3731 Chevrolet · Mar 2017 | Sonic 2012–2014 | A cracked air cleaner outlet duct lets air bypass the MAF sensor. Dealers replace the duct at no charge. |
| PIP5686A GMC · Nov 2019 | Sierra 1500 2019–2020 | Rough idle and an air leak noise from a PCV tube that was installed wrong or damaged. |
| 15317 Cadillac · Aug 2015 | Cts 2011 | A cracked PCV hose on the CTS-V can set P0171 and P0174. Dealers replace the hose. |
| 09-025-25 Ram · Aug 2025 | 1500 2025 | A disconnected purge line causes lean codes, misfires and a hissing noise. The purge hose and tube are replaced. |
| TSB 20-2324 Ford · Sep 2020 | F-150 2018 | On the 2.7L EcoBoost, engine software or direct injection injectors leaking down can set lean and misfire codes. |
| PIP3711B Buick · Sep 2015 | Rainier 2006 | P0171 with no drivability problem, often from an intake leak or air filter issue between the MAF sensor and the throttle body. |
Vehicles with the most P0171 reports
P0171 by make
Owner complaints and service bulletins that mention P0171, top 10 makes
Owner complaintsService bulletins
Bulletins are counted per make and bulletin number; one bulletin can cover several models and years.
| Make | Model | Complaints |
|---|---|---|
| Chevrolet | Cruze | 13 |
| Chevrolet | Sonic | 8 |
| Ford | Windstar | 7 |
| Toyota | Corolla | 5 |
| Toyota | Camry | 5 |
| Chevrolet | Traverse | 5 |
| Ford | Focus | 5 |
| Chevrolet | Equinox | 4 |
| Toyota | RAV4 | 4 |
| GMC | Terrain | 4 |
Chevrolet leads owner complaints, and its top two models are the Cruze and the Sonic. Both were sold with the 1.4L turbo engine whose PCV valve sits inside the camshaft cover, the part covered by the two GM special coverage programs in the bulletins above. If you own one of these cars and see P0171, ask a GM dealer about the coverage before paying for a diagnosis.
P0171 complaints by model year
Model years 2005–2024; peak year 2013 highlighted
Unlike misfire codes, P0171 complaints are spread over many model years without a sharp peak. Owners who gave their mileage reported a median of 85,312 miles.
Symptoms owners report with P0171
What owners describe alongside P0171
Share of 158 NHTSA owner complaints that mention P0171
Counted by phrases in the complaint text. One complaint can mention several things.
The check engine light shows up in most complaints. Stalling is the next most common report, followed by a loss of power. A lean engine idles poorly and can die when it slows to a stop, which is when extra air has the biggest effect.
Finding the cause of a P0171
Lean codes are easiest to solve with live data and a smoke machine. A sensible order:
- Read long-term and short-term fuel trim at idle, then with the engine held at 2,500 rpm. Trim that is high at idle and closer to normal at higher speed points to an air leak. Trim that stays high or gets worse with speed points to the MAF sensor or fuel delivery.
- Smoke test the intake. Smoke pushed into the intake escapes through cracks and loose couplers that are hard to find by ear.
- Check the PCV valve and its hoses. On some engines the valve is built into the valve cover, and a torn diaphragm can hiss at idle.
- Compare the MAF sensor reading with what the engine size and speed should produce. A dirty sensor can sometimes be cleaned with a cleaner made for MAF sensors.
- Measure fuel pressure, and on direct injection engines check for injectors that leak or stick.
- Ask a dealer to check your VIN for open bulletins and special coverage before buying parts.
Can you keep driving with P0171?
For a short time, usually yes. The car often feels normal at first. Over weeks, a lean mixture makes the engine run hotter and can turn into misfires, and stalling at low speed is a safety issue in traffic. A flashing check engine light means a misfire has started. Shut the engine off as soon as it is safe.
P0171 questions owners ask
What is the most common cause of P0171?
Air entering the engine after the mass airflow sensor is the classic cause, through a vacuum leak or a cracked intake duct. In manufacturer bulletins the single most common fix is a software update, with PCV problems second.
How do I fix a P0171 engine code?
Look for an air leak first, ideally with a smoke test of the intake. Then check the MAF sensor reading and fuel pressure. Ask a dealer whether your VIN has an open bulletin or special coverage, because several GM lean problems are repaired free.
Can I still drive my car with a P0171 code?
Usually yes for a short time if it idles and accelerates normally. A lean engine runs hotter and can start to misfire, so get it checked soon, and stop driving if the check engine light begins to flash.
Can a dirty MAF sensor cause P0171?
Yes. If the sensor under-reads airflow, the computer injects too little fuel and has to correct upward, the same pattern an intake leak produces. A contaminated sensor tends to show a lean condition at higher engine speed, while a leak shows up most at idle.
What does it mean when P0171 and P0174 appear together?
Both banks are running lean, so the cause is probably shared by the whole engine. Low fuel pressure and a faulty MAF sensor are typical examples. A fault that only affects one side of the engine would usually set just one of the two codes.
Can a bad oxygen sensor cause P0171?
It can, but less often than people think. The front sensor reports what it sees, and a lean reading is usually correct. Replace it only after the air leak and MAF checks come back clean.