AutoDiag

P0171

System Too Lean (Bank 1)

Moderate· Fix soonIntermediatePowertrainGeneric, all makes

What it means

The ECU has had to add much more fuel than normal on bank 1 and has reached its limit. "Lean" means too much air or too little fuel. The ECU is not saying a part has failed; it is saying the mixture is wrong and it cannot correct it. Bank 1 is the side with cylinder 1 (the only bank on 4-cylinder engines).

Is it safe to drive?

Usually safe to drive short-term, but a lean engine can misfire, hesitate and run hot. Fix it within a few weeks to protect the engine and catalytic converter.

Symptoms

  • Check engine light on
  • Rough or high idle
  • Hesitation or stumbling on acceleration
  • Misfires in some cases
  • Hissing noise from the engine bay (vacuum leak)

Probable causes

Most likely first

  1. 1Vacuum leak: cracked hose, leaking intake gasket, PCV valve or hose
  2. 2Dirty or failing MAF sensor under-reading airflow
  3. 3Unmetered air leak in the intake boot between the MAF and throttle body
  4. 4Low fuel pressure: weak fuel pump, clogged fuel filter, faulty regulator
  5. 5Clogged or leaking fuel injectors
  6. 6Exhaust leak before the upstream O2 sensor
  7. 7Leaking brake booster diaphragm

How to diagnose it

Work through these in order before buying parts

  1. 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
  2. 2Check short-term and long-term fuel trims for bank 1 at idle and at 2,500 RPM. If trims are high at idle but improve at higher RPM, suspect a vacuum leak. If they get worse with RPM, suspect fuel delivery or the MAF.
  3. 3If bank 2 is also lean (P0174), look for a shared cause: MAF, fuel pressure or a large vacuum leak.
  4. 4Listen for hissing and do a smoke test of the intake. Spraying carb cleaner near suspected leaks and listening for an idle change also works (keep away from hot exhaust).
  5. 5Clean the MAF sensor and check its reading against expected airflow.
  6. 6Test fuel pressure and volume against specification.
  7. 7Check for an exhaust leak ahead of the O2 sensor.
  8. 8Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

  • Repair the vacuum leak (hoses, intake gasket, PCV)
  • Clean or replace the MAF sensor
  • Replace the fuel pump or filter
  • Clean or replace injectors
  • Repair exhaust leaks

Guides

Diagnostics

How to read a check engine code (and what to do next)

A plain-English walkthrough of plugging in an OBD-II scanner, reading the code, and deciding whether it is safe to keep driving.

This information is general guidance for all makes. Exact causes, specifications and procedures vary by vehicle, so confirm with your service manual or a qualified mechanic before replacing parts.