Fuel and air metering
P0171 — System Too Lean (Bank 1)
The ECU is adding more fuel than normal to keep the mixture balanced on bank 1, but it still reads lean. That means too much air or too little fuel is reaching the cylinders.
Common causes
- Vacuum or intake air leak (hoses, intake gasket, PCV)
- Dirty or faulty MAF sensor
- Weak fuel pump, clogged filter or low fuel pressure
- Clogged or leaking fuel injectors
- Exhaust leak before the oxygen sensor
Symptoms
- Rough idle or stalling
- Hesitation and power loss
- Misfire on acceleration
- Check engine light
How to check
- Look at fuel trims: if long-term trim is very positive at idle and normalizes at higher rpm, suspect a vacuum leak
- Inspect intake hoses, PCV system and intake gasket; smoke-test if needed
- Check MAF reading and clean the sensor with MAF cleaner
- Measure fuel pressure and check injectors; check the exhaust for leaks
Can you keep driving?
Usually drivable for a short time, but a lean mixture can overheat the engine and damage the catalytic converter. Do not ignore it.
Related codes
Frequently asked questions
- What does P0171 mean?
- The ECU detects that bank 1 has too much air or too little fuel and cannot fully correct it.
- Is a vacuum leak the most common cause of P0171?
- Yes, along with a dirty MAF sensor. Check those first.
General reference information. It does not replace a diagnosis on your own vehicle. The same code can have different causes depending on make, model year and engine; check the manufacturer's documentation or ask a technician with a scan tool.