P0171: System Too Lean (Bank 1)

What it means

Long-term fuel trim on bank 1 is at the upper limit — the ECU is adding more fuel than baseline to maintain stoichiometric mixture but cannot reach target. Indicates unmetered air entering the intake or insufficient fuel delivery.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The engine control module compares oxygen-sensor feedback with its target mixture and adjusts fuel delivery for bank 1. P0171 sets when fuel corrections on bank 1 reach their lean limit, indicating too much air or too little fuel for that bank.

Common causes

Most likely first.

1. Vacuum leak in intake or PCV system
2. MAF sensor contaminated, under-reading
3. Fuel pressure below specification
4. Fuel injectors partially clogged

Symptoms

• Check engine light on
• Rough idle, possible stalling
• Hesitation or hard cold start
• Power loss under load

How to diagnose it

1. Read freeze-frame data and short- and long-term fuel trims to see whether the lean correction is strongest at idle or under load.
2. Inspect bank 1 intake and PCV hoses, manifold connections, and nearby vacuum lines for cracks or leaks.
3. Check the MAF sensor and its intake duct for contamination, damage, or air entering downstream of the sensor.
4. Test fuel pressure against the manufacturer's specification and review injector operation if the lean condition persists under load.
5. If the fault remains isolated to bank 1, check its injectors and exhaust for leaks that could distort oxygen-sensor feedback.

Common fixes

• Reconnect or replace a leaking bank 1 vacuum or PCV hose.
• Seal an intake leak affecting bank 1.
• Clean or replace the MAF sensor if inspection and testing support it.
• Restore fuel pressure or service restricted injectors when tests confirm a fuel-delivery problem.

Can I keep driving?

If the engine stalls, runs very roughly, or loses power, avoid driving until the lean condition is diagnosed. Otherwise, drive gently and watch for worsening symptoms; a persistent lean mixture can impair running and increase emissions.