P0171: System Too Lean (Bank 1)
The engine on bank 1 is getting too much air or too little fuel, and the computer has run out of room to correct it.
Severity: 2 of 4
Usually drivable in the short term if the car runs well and the light is steady. Get it fixed soon, because a lean engine can misfire and run hot. If you get misfire codes or the light flashes, park it.
Estimate range covers the repairs listed below; see the cost breakdown. A flashing check engine light, a hot temperature gauge, or brakes that feel different mean stop driving and get help.
What P0171 means
Your engine computer constantly adjusts how much fuel it injects based on the front oxygen sensor. These adjustments are called fuel trims. When the computer has to add a lot of extra fuel for a long time and still cannot reach the right mixture, it sets P0171.
Lean means extra air or missing fuel. Unmetered air leaking in after the airflow sensor is the most common reason.
Running lean can cause hesitation, misfires, and higher combustion temperatures.
Common causes (cheapest likely first)
- Vacuum leak. Cracked hoses, a leaking PCV hose, or a loose intake boot. Listen for hissing at idle.
- Dirty mass airflow (MAF) sensor. A coated sensor under reports airflow. Cleaning it with MAF specific cleaner is cheap.
- Leaking intake manifold gasket. Common on older engines, especially with plastic manifolds.
- Weak fuel pump or clogged fuel filter. Low pressure means less fuel than the computer expects.
- Exhaust leak before the front oxygen sensor. Outside air pulled into the exhaust makes the sensor read lean.
- Faulty front oxygen sensor. Less common but possible.
Symptoms
- Steady check engine light
- Rough or surging idle
- Hesitation on acceleration
- Possible misfire codes
- Hard starting
DIY difficulty: Easy to moderate
Inspecting hoses and cleaning the MAF sensor are simple. Finding small vacuum leaks is easier with a smoke tester, which many shops use.
Tools: OBD-II scanner with fuel trim data, MAF sensor cleaner, Screwdrivers and pliers, Smoke tester (optional), Fuel pressure gauge
Estimated repair cost
This is an estimate, built from typical parts price ranges plus labor hours times an assumed shop rate of $110 to $190 per hour. Actual prices vary by vehicle, region, and shop. Diagnosis fees are often extra.
| Repair | Parts | Labor | Estimate (low / high) |
|---|---|---|---|
| Vacuum or PCV hose | $10 to $80 | 0.3 to 1.0 h | $10 + 0.3 × $110 = $43 $80 + 1.0 × $190 = $270 |
| MAF sensor cleaning or replacement | $10 to $250 | 0.2 to 0.5 h | $10 + 0.2 × $110 = $32 $250 + 0.5 × $190 = $345 |
| Intake manifold gasket | $20 to $150 | 1.0 to 4.0 h | $20 + 1.0 × $110 = $130 $150 + 4.0 × $190 = $910 |
| Fuel pump | $120 to $600 | 1.5 to 3.5 h | $120 + 1.5 × $110 = $285 $600 + 3.5 × $190 = $1,265 |
Watch a repair video
System Too Lean: Code P0171 or P0174 (1A Auto). Third-party video, loads from YouTube (privacy-enhanced mode) only when you press play. Procedures vary by vehicle. Search YouTube for “P0171 fix” videos.
P0171 by make
Brand specific notes: Ford, Chevrolet, Toyota, Nissan, Volkswagen, Subaru. Other makes: the code works the same way; see all makes.
Repair manuals
Step-by-step service manuals with torque specs and wiring diagrams:
Haynes manuals Chilton manuals
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Related codes
Check for recalls
Before paying for a repair, enter your VIN in the NHTSA recall lookup. Emissions parts may also be covered by a federal emissions warranty; check your warranty booklet.
FAQ
What is the most common cause of P0171?
A vacuum leak, followed by a dirty MAF sensor.
Can P0171 damage my engine?
Mild lean running is not instantly harmful, but sustained lean conditions can cause misfires and overheating of internal parts.
Why do I have both P0171 and P0174?
Both banks are lean, so look for something shared: a large vacuum leak, the MAF sensor, or low fuel pressure.