P0171 — System Too Lean (Bank 1)
P0171 means there's too much air (or not enough fuel) in the air-fuel mixture on Bank 1. It's one of the most common check engine codes and has many possible causes — from a $15 gas cap to a $500 MAF sensor.
What Does 'Running Lean' Mean?
A lean condition means the engine is getting more air than fuel relative to the ideal ratio (14.7:1 air:fuel). The ECU tries to compensate by adding more fuel (positive fuel trim), but if the correction is too large, it sets P0171. Running lean long-term causes engine knock, overheating, and can damage valves and pistons.
Common Causes (Ranked)
- Vacuum leak — cracked hose, failed intake gasket, or brake booster hose (most common, often free to fix)
- Dirty or failing MAF sensor — cleaning with MAF cleaner spray ($10) fixes it often
- Clogged fuel injectors — not delivering enough fuel
- Weak fuel pump — low pressure causes lean condition at high load
- Dirty or clogged fuel filter
- Failing O2 sensor — sending incorrect readings to ECU
P0171 + P0174 Together
If both P0171 (Bank 1) and P0174 (Bank 2) are set simultaneously, the lean condition affects both banks of the engine. This almost always points to a large vacuum leak (affecting all cylinders) or a failing MAF sensor (which affects the whole engine, not just one bank). Cylinder-specific causes like injectors would typically only affect one bank.
Repair Cost
- MAF sensor cleaning (DIY): $10 in cleaner spray
- MAF sensor replacement: $150–$320
- Vacuum leak repair: $100–$400 depending on location
- Fuel injector cleaning service: $100–$200
- Fuel pump replacement: $400–$900
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