P0171: System Too Lean Bank 1
What Does P0171 Mean?
P0171 is a diagnostic trouble code indicating: System Too Lean Bank 1. This code relates to the fuel and air metering system. It has been reported in NHTSA complaints across 34 different vehicle models.
P0171 sets when Bank 1 fuel trims stay positive long enough that the ECU reaches its correction limit. The computer is adding fuel because the measured mixture looks lean, and the trim pattern tells you where to start. The highest-probability path is still lean-condition diagnosis: unmetered air after the MAF, an under-reporting MAF, weak fuel delivery, or a biased Bank 1 feedback sensor after the basics check out.
Use freeze-frame plus live fuel-trim data before replacing parts. High positive STFT and LTFT at idle that improve as RPM rises usually point to a vacuum leak after the MAF. High positive trims at idle and cruise point more toward an under-reporting MAF, low fuel pressure, restricted injectors, or an upstream O2 feedback problem that keeps the sensor reading lean.
**Lean-diagnosis-first workflow:** (1) Save freeze-frame, then compare STFT and LTFT at hot idle, 2500 RPM, and steady cruise. (2) Smoke-test the intake tract, PCV hoses, brake-booster hose, and purge lines for vacuum leaks. (3) Inspect and clean the MAF, then verify its reading is plausible for engine size and load. (4) Check fuel pressure, fuel volume, and injector balance if trims stay positive under load. (5) If the air path and fuel delivery check out, verify the Bank 1 upstream O2 or air-fuel sensor is not biased lean before replacing it.
Bank-to-bank comparison matters. If P0171 appears with P0174, suspect a shared air leak or weak fuel supply first. If only Bank 1 sets, look harder for a bank-specific intake leak, injector issue, or exhaust leak ahead of the Bank 1 upstream sensor.
Symptoms of P0171
- ⚠️ check engine light
- ⚠️ rough idle
- ⚠️ hesitation or stumbling on acceleration
- ⚠️ poor fuel economy
- ⚠️ possible misfires
Common Causes
- 🔍 Vacuum leak (intake manifold gasket, hoses, PCV valve)
- 🔍 Dirty or faulty MAF sensor
- 🔍 Weak fuel pump or clogged fuel filter
- 🔍 Leaking or clogged fuel injectors
- 🔍 Exhaust leak before O2 sensor
P0171 Reports by Year
Real Owner Reports
From NHTSA complaint database — actual owner descriptions.
"WE WHERE DRIVING TO THE STORE AND ON THE WAY THE CHECK ENGINE CAME ON, SO WE PULLED INTO THE LOCAL AUTOZONE AND THEY RAN A SCAN FOR FAULT CODES. IT HAD A CODE P0171 WHICH THEY SAID IT COULF 1 OF SEVERAL POSIBILITIES AND SHOULD HAVE A REPAIR SHOP PERFORM A PROPER CHECK TO BE SURE WHAT IS ACTUALLY GOI"
— Toyota Camry owner, 07/02/2014
"THE FIRST INDICATION WAS THE CRL CAME ON WITH CODES P0171 AND THEN P0174. WHEN PRESSING THE BRAKE PEDAL DOWN, THERE IS A HISSING NOISE AND THE BRAKE PEDAL WOULD BASICALLY GO ALL THE WAY TO THE FLOOR. IT SOUNDS AS IF THIS HAS HAPPENED QUITE A BIT WITH THIS MODEL AND SHOULD BE LOOKED INTO. THE DEAL"
— Toyota Camry owner, 10/20/2012
"CHECK ENGINE LIGHT CAME ON AND THE PERFORMANCE OF THE VEHICLE IS BAD. BAD IDLE, WEAK ENGINE, JUST THE CAR IS NOT PERFORMING PROPERLY. I USED THE SCANNER TO SEE THE CODE AND I HAD MORE THAN ONE . P0171 P0505 P117B P0441 P0455 . FUEL/AIR MIXTURE IS LEAN . AND I HEAR THE FUEL PUMP MAKING THAT NOISE THA"
— Toyota Camry owner, 06/20/2020
Source: NHTSA Complaints Database
Most Reported Vehicles for P0171
Based on NHTSA owner complaint data.
| # | Vehicle | Reports |
|---|---|---|
| 1 | CHEVROLET CRUZE | |
| 2 | FORD FOCUS | |
| 3 | TOYOTA CAMRY | |
| 4 | TOYOTA COROLLA | |
| 5 | TOYOTA RAV4 | |
| 6 | GMC TERRAIN | |
| 7 | CHEVROLET EQUINOX | |
| 8 | CHEVROLET MALIBU | |
| 9 | TOYOTA HIGHLANDER | |
| 10 | FORD F-150 | |
| 11 | CHEVROLET TRAVERSE | |
| 12 | SUBARU WRX | |
| 13 | VOLKSWAGEN GTI | |
| 14 | MERCEDES-BENZ C-CLASS | |
| 15 | TOYOTA TACOMA |
P0171 Reports by Vehicle
Detailed NHTSA complaint analysis for each vehicle model.
Diagnostic Tips
- Save freeze-frame data before clearing anything, then compare STFT and LTFT at hot idle, 2500 RPM, and light cruise.
- If fuel trims are most positive at idle, smoke-test for vacuum leaks at the intake manifold, PCV hoses, brake-booster hose, and purge plumbing.
- If trims stay lean across the RPM range, check MAF readings against expected load and verify fuel pressure and fuel volume under load.
- Watch the Bank 1 upstream O2 or air-fuel sensor for a biased lean signal only after the air path and fuel supply check out.
- After each repair step, recheck fuel trims and complete a drive cycle so the monitor can confirm the lean condition is gone.
- If the code keeps returning on high-incidence platforms (for example CHEVROLET CRUZE), check TSB patterns and wiring/connector fitment before major part replacement.
Common Mistakes to Avoid
- ❌ Skipping freeze-frame review and losing the exact load and RPM conditions when the fault set.
- ❌ Replacing the MAF, pump, injectors, or upstream O2 sensor before proving whether trims change at idle versus cruise.
- ❌ Chasing ignition or timing parts without fuel-trim evidence that supports a lean diagnosis.
- ❌ Clearing P0171 and returning the car before trims normalize and monitors complete.
Frequently Asked Questions
What's the most common cause of P0171?
Can P0171 and P0174 appear together?
Is P0171 dangerous?
Will P0171 cause rough idle?
What are normal fuel trim values?
Reported Repair Costs for P0171
Based on 4 owner-reported repair costs from NHTSA complaints.
⚠️ These are owner-reported costs, not estimates. Actual costs vary by location, labor rates, and root cause. Source: NHTSA complaints database.
What To Do Next
Possible Fixes
- 🔧 Inspect and repair vacuum leaks
- 🔧 Clean MAF sensor
- 🔧 Replace fuel filter
- 🔧 Replace fuel pump
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1
Find your vehicle above
Click your make and model for real owner reports and common causes specific to your vehicle.
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2
Check for recalls
Go to nhtsa.gov/recalls and enter your VIN. Free repairs if covered.
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Get a professional diagnosis
A code alone doesn't identify the exact failed part. A diagnostic ($50–$150) pinpoints the root cause.