P0171 and P0174: system too lean
The engine computer has had to add much more fuel than usual to keep the mixture right, and it has run out of room to add more. P0171 is bank 1, P0174 bank 2.
SeverityModerateUsually not urgent today, but it can get worse if left.
Can you keep driving?Drive gently, check it soonWhat does P0171 mean?
The upstream oxygen sensor tells the computer whether the mixture is rich or lean, and the computer corrects it with fuel trim. When the long-term correction stays at the top of its range, either more air is getting in than the computer measured, or less fuel is arriving than it asked for. Both banks lean at once points to a cause the whole engine shares.
What causes P0171?
- A vacuum leak: a cracked hose, a leaking intake gasket or a split breather (PCV) hose
- A dirty or faulty mass air flow sensor that under-reads the air
- Low fuel pressure from a weak pump, a clogged filter or a faulty regulator
- Restricted or leaking injectors
- An exhaust leak ahead of the upstream oxygen sensor that lets outside air in
What to check first for P0171
- See whether the correction is worst at idle, typical of a vacuum leak, or under load, typical of fuel supply
- Inspect the intake hoses and the breather system for cracks, and listen for hissing
- Clean or test the mass air flow sensor
- Have the fuel pressure measured
Can you drive with P0171?
You can usually drive to a workshop, but do not leave it for weeks. A lean engine runs hotter, can hesitate or misfire, and a lean mixture over time can damage the valves and the catalytic converter.
Since May 2018 an engine warning light that stays on is a major defect at the MOT, so the car fails until the fault is fixed.
Codes on this page
- P0171System Too LeanBank 1
- P0174System Too LeanBank 2
Related codes
Checked on · Rafael Soares
These are the generic meanings every manufacturer shares. Each brand adds its own detail, so your car's handbook, its service information and a qualified mechanic have the last word. OdoKeep has no connection with any manufacturer, and the app does not read codes from the car.
