# P0171 — System Too Lean (Bank 1)

> ClearTheCode diagnostic code reference (not OEM service data).

Source: https://clearthecode.com/codes/P0171

**System:** Powertrain · Fuel & Air Metering

**Severity:** moderate

## Description

The engine is running with too much air relative to fuel on Bank 1. Top causes: vacuum leak, dirty MAF sensor, weak fuel pump, clogged fuel filter, faulty oxygen sensor, leaking intake manifold gasket.

## Diagnostic snapshot

- **Difficulty:** 6/10 — Intermediate diagnostics
- **Est. repair cost:** $40 – $450
- **Common symptoms:** Check engine light (MIL), Rough idle, Poor fuel economy

## Diagnostic workflow

1. Confirm P0171 with a live scan — note pending vs stored and freeze frame data.
2. Verify reported symptoms: Check engine light (MIL), Rough idle, Poor fuel economy.
3. Inspect wiring/connectors and related sensors before replacing modules.
4. Most likely fixes: Diagnose and repair vacuum / air leak; Replace oxygen sensor(s); Clean or replace MAF sensor.
5. Clear codes and road-test; re-scan after two drive cycles if the monitor must set.

## Likely fixes

1. **Diagnose and repair vacuum / air leak** (moderate)
2. **Replace oxygen sensor(s)** (easy)
3. **Clean or replace MAF sensor** (easy)

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