CarFix Answers

P0171: System Too Lean Bank 1

Severity: medium

What it means: The engine computer is adding more fuel than it should need on bank 1, because it sees too much air or too little fuel.

Safe to drive? Usually OK for short trips, but have it checked. Running lean for a long time can cause misfires and engine damage.

Common symptoms

Likely causes, ranked

  1. Vacuum or intake air leak (cracked hose, PCV, intake gasket)DIY
  2. Dirty or failing mass air flow sensor$323DIY

    CarMD 2024 national average about $323

  3. Weak fuel pump, clogged filter or low fuel pressureShop
  4. Clogged fuel injectors$464Shop

    CarMD 2024 national average about $464

  5. Exhaust leak ahead of the upstream O2 sensorShop
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Answers that mention P0171

What it means: The engine computer is adding more fuel than it should need on bank 1, because it sees too much air or too little fuel.

Severity: Medium. Safe to drive? Usually OK for short trips, but have it checked. Running lean for a long time can cause misfires and engine damage.

DIY or shop?

A smoke test for vacuum leaks and MAF cleaning are common DIY steps. Fuel pressure testing is usually a shop job.

Before you replace parts

  1. Read all stored codes and freeze-frame data. Fix misfire, fuel or sensor codes before chasing emissions codes.
  2. Check the simple things first: connectors, vacuum hoses, fluid levels and the gas cap.
  3. Clear the code only after a repair, then confirm it stays off after a few drives.
  4. Check your VIN for recalls and service bulletins.

Safety / mechanic note

Verify with a mechanic if you notice drivability changes, warning lights or overheating. A code points to a circuit or system, not always a specific part.

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Sources