P0171: System Too Lean (Bank 1)
The engine computer is adding a lot of extra fuel to bank 1 to keep the air-fuel mix right. That means extra air is getting in, or not enough fuel is getting there.
Green, yellow, red: three ways P0171 can look
The same code can be a quick fix or a reason to pull over. Match your situation to the closest example.
Keep an eye on it
Light is on but the car drives normally. It still needs attention, but it can wait a few days.
Schedule a repair
Rough idle, hesitation, or the code keeps coming back. Schedule diagnosis. A lean engine runs hotter and can start misfiring.
Stop and get help
Lean code with misfires, a flashing light, or a strong fuel smell. Stop driving until it's fixed.
Symptoms you might notice
- Usually none at first
- Rough idle
- Hesitation
- Check engine light
Common causes, cheapest first
- Vacuum leak (cracked hose, intake gasket, PCV)
- Dirty or failing MAF sensor
- Weak fuel pump or clogged fuel filter
- Exhaust leak ahead of the O2 sensor
- Clogged fuel injectors
What you can check before calling
- Listen for a hiss around the intake at idle
- Look for cracked vacuum hoses
- Note if it's worse at idle or under load
A code points to a system, not a part. Replacing parts based on the code alone is how people spend money without fixing the problem. Testing confirms the cause first.
Want the full story? Read our guide: P0171 "system too lean": causes and how to find the real one.
Generic codes (second character 0) mean the same thing on every vehicle. Some manufacturers add their own details, so check service information for your exact vehicle.
Related codes
More fuel & air mixture codes
Look up another code
Cummins fault code (like 1922), J1939 SPN and FMI (like SPN 3251 FMI 0), or an OBD-II code (like P0420).
Questions
What does P0171 mean?
P0171 means "System Too Lean (Bank 1)". The engine computer is adding a lot of extra fuel to bank 1 to keep the air-fuel mix right. That means extra air is getting in, or not enough fuel is getting there.
Can I drive with code P0171?
It depends on what the vehicle is doing. Green: Light is on but the car drives normally. It still needs attention, but it can wait a few days. Yellow: Rough idle, hesitation, or the code keeps coming back. Schedule diagnosis. A lean engine runs hotter and can start misfiring. Red: Lean code with misfires, a flashing light, or a strong fuel smell. Stop driving until it's fixed.
What causes P0171?
The most common causes are: vacuum leak (cracked hose, intake gasket, PCV); dirty or failing MAF sensor; weak fuel pump or clogged fuel filter.
Is P0171 a serious code?
It can be. Lean code with misfires, a flashing light, or a strong fuel smell. Stop driving until it's fixed.
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