P0171: System Too Lean (Bank 1)

Updated November 12, 2026

The engine computer is adding more fuel than normal to keep the air-fuel mixture right on bank 1, and it has reached its limit. In other words, the engine is getting too much air or not enough fuel.

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🔧 Want to check this yourself? An OBD2 scanner reads the code; models with live data also show the sensor readings used in the checks below. See code readers and scanners from $24.99 ↓ · Which scanner do I need?

Common causes

Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.

Symptoms

Is it safe to drive?

Often drivable short term, but a lean engine can run hot and misfire, so fix it soon.

How to check it with a scanner

  1. Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
  2. Look at long-term fuel trims in live data. High positive numbers (well above +10%) point to a lean condition.
  3. With the engine running, keep hands and loose clothing away from belts and the fan, and listen for hissing around the intake and check vacuum hoses; a smoke test finds leaks fast.
  4. Check MAF readings at idle and under load, and clean the sensor with MAF cleaner before replacing it.
  5. After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.

A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.

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Questions

What does P0171 mean?

P0171 is the generic OBD2 code for "System Too Lean (Bank 1)." The engine computer is adding more fuel than normal to keep the air-fuel mixture right on bank 1, and it has reached its limit. In other words, the engine is getting too much air or not enough fuel.

Is it safe to drive with P0171?

Often drivable short term, but a lean engine can run hot and misfire, so fix it soon.

How do I check P0171 myself?

Read the code and freeze-frame data with an OBD2 scanner, then: Look at long-term fuel trims in live data. High positive numbers (well above +10%) point to a lean condition. With the engine running, keep hands and loose clothing away from belts and the fan, and listen for hissing around the intake and check vacuum hoses; a smoke test finds leaks fast. Check MAF readings at idle and under load, and clean the sensor with MAF cleaner before replacing it.

Related codes

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