P0171 — Fuel Trim System Too Lean (Bank 1)
Short answer: P0171 (Bank 1 too lean) is often Green for short trips if the light is steady and the car feels normal. Roughness or weak power → Yellow. Flashing light, stalling, or smoke → Red.
(Asia/Shanghai)
Can I keep driving?
| Color | When it applies |
|---|---|
| Green (conditional) | Steady light, no stumble or hesitation, and the car otherwise feels normal. Short trips while you schedule diagnosis are often acceptable. |
| Yellow | Rough idle, surging, or weak power. Drive gently to a shop soon and avoid high load. |
| Red — stop when safe | Flashing check engine light; stalling; overheating; smoke; strong fuel smell; or brake / ABS / airbag / steering-assist warnings. |
What P0171 means
P0171 means the computer can no longer add enough fuel to correct a lean condition on Bank 1 — too much air, too little fuel, or a sensor picture that looks that way. Bank 1 includes cylinder 1.
The computer uses upstream oxygen or air-fuel sensor feedback and long/short fuel trims to keep mixture in range. When trim hits its limit on the “add fuel” side, P0171 can set and the check engine light can come on.
A lean code is not an automatic order to replace the oxygen sensor. Vacuum leaks and air-metering problems are classic root causes. The sensor may be reporting truthfully.
Common causes
- Vacuum, PCV, or intake-boot leaks (unmetered air)
- Dirty or faulty mass air flow (MAF) sensor
- Low fuel pressure or a restricted fuel filter
- Exhaust leak before the upstream sensor (can skew readings)
- Stuck or failing fuel injector
- Faulty upstream O2 / A/F sensor — after cheaper and more common causes are considered
When to see a technician
A technician will usually look at fuel trims at idle versus higher RPM, inspect for intake leaks, and verify fuel delivery and sensor operation. Smoke testing for vacuum leaks is a common shop method — described here only as context, not as a guaranteed home procedure.
This page does not diagnose your vehicle and does not promise that cleaning a MAF or tightening a hose will clear P0171.
Related codes
- P0174 — lean on Bank 2
- P0172 — rich on Bank 1
- P0300 — misfire
- P0420 — catalyst efficiency
- P0171 Honda Civic · P0171 Ford F-150 — model notes
FAQ
Can I drive with a P0171 code?
Often yes for short trips if the light is steady and the car feels normal. Roughness or weak power: drive gently to a shop. Flashing light or stalling: stop when safe.
What does P0171 mean?
P0171 means Bank 1 fuel trim looks too lean—too much air, too little fuel, or a sensor picture that looks that way.
Is P0171 serious?
It can be if drivability is poor or a misfire develops. Even when it feels fine, diagnose soon rather than ignoring long-term lean operation.
Page disclaimer
Not certified repair advice. This page explains a generic OBD-II code in plain English and a cautious “can I keep driving?” guide. It does not diagnose your exact vehicle and does not guarantee any repair. If the light is flashing, the engine overheats, or you have brake, steering, airbag, or severe drivability problems, stop when safe and get professional help. Details are rewritten from public DTC definitions—confirm with a scanner and a qualified technician.