P0171: System Too Lean (Bank 1)
The engine computer is adding a lot more fuel than normal on bank 1 because it is seeing too much air and not enough fuel.
What P0171 means
Engines run best at about 14.7 parts air to 1 part fuel. The engine computer reads the oxygen sensors and trims the fuel up or down to stay there. When the front oxygen sensor keeps reporting lean exhaust, the computer adds fuel, and that correction is called long term fuel trim. RepairPal explains that a healthy engine stays within a couple of percent, while P0171 sets when the trim has climbed to roughly 15 to 35 percent and the computer cannot add any more. Bank 1 is the side with cylinder 1; a four cylinder only has bank 1. The most common cause is air sneaking in after the MAF sensor, so the computer never counts it. Shops read fuel trims at idle and at 2,500 to 3,000 RPM. If the trim improves at higher RPM, it is likely a vacuum leak; if it gets worse, fuel delivery or the MAF is suspect. A smoke test and fuel pressure test usually finish the job.
Common symptoms
- Check engine light on
- Rough or hunting idle, worse when warm
- Hesitation or coughing on acceleration
- Lack of power
- Occasional misfire codes along with it
Most common causes
- Vacuum leak from a cracked hose, PCV hose or intake gasket
- Dirty or failing mass airflow sensor
- Weak fuel pump or clogged fuel filter
- Clogged fuel injectors
- Exhaust leak ahead of the front oxygen sensor
- Engine computer software that needs an update
What to check yourself first
With the engine idling, listen around the intake for a hiss and check every vacuum and PCV hose for cracks. Make sure the air intake boot between the MAF sensor and throttle body is not split. Cleaning the MAF sensor with dedicated MAF cleaner is a cheap first step. Do not replace oxygen sensors first; RepairPal lists them as a common misdiagnosis.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Smoke test for vacuum leaks | $61-$90 | A smoke test is the fastest way to find a hidden vacuum leak, the top cause of P0171. |
| Clean the MAF sensor yourself | $15-$20 | A can of CRC MAF cleaner costs about $17 at retail. Spray only the sensor wires and let it dry before reinstalling. |
| Replace mass airflow (MAF) sensor | $274-$381 | If cleaning does not help and live data shows low airflow readings. |
| Replace intake manifold gasket | $683-$834 | When the smoke test finds the leak at the intake manifold gasket. |
| Replace fuel pump | $1,344-$1,513 | Only after a fuel pressure test confirms low pressure. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0174 System Too Lean (Bank 2)
- P0172 System Too Rich (Bank 1)
- P0101 Mass or Volume Air Flow "A" Circuit Range/Performance
- P0300 Random/Multiple Cylinder Misfire Detected
- P0507 Idle Air Control System RPM Higher Than Expected