P0171 — System Too Lean (Bank 1)

What it means

The engine control module has detected that long-term fuel trim on bank 1 has reached its upper limit: the mixture contains too much air relative to fuel. The module keeps extending injection time to compensate, and this excessive positive correction is stored as a fault. It typically shows up as unstable idle, hesitation under load and increased fuel consumption.

Likely causes (most common first)

1. Unmetered air leak in the intake tract after the mass air flow sensor: cracked hoses, leaking intake manifold gasket, faulty PCV system
2. Contaminated or drifted mass air flow sensor underreporting the incoming air
3. Low fuel pressure: weak fuel pump, clogged fuel filter, faulty pressure regulator
4. Clogged or restricted injectors on bank 1
5. Exhaust leak ahead of the upstream oxygen sensor drawing in fresh air
6. Aged or slow upstream oxygen sensor reporting the mixture incorrectly

Can you drive with it?

Short trips are possible, but prolonged driving on a lean mixture overheats pistons and valves, so have it diagnosed promptly.