P0171 (generic)
System Too Lean (Bank 1)
System: Powertrain · Fuel & Air Metering
Severity: moderate · Inspection: often fails
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 P0171
- Difficulty
- 6/10 — Intermediate diagnostics
- Est. repair cost
- $40 – $450 (parts + typical shop labor)
- Common symptoms
- Check engine light (MIL)
- Rough idle
- Poor fuel economy
Diagnostic workflow
- Confirm P0171 with a live scan — note pending vs stored and freeze frame data.
- Verify reported symptoms: Check engine light (MIL), Rough idle, Poor fuel economy.
- Inspect wiring/connectors and related sensors before replacing modules.
- Most likely fixes: Diagnose and repair vacuum / air leak; Replace oxygen sensor(s); Clean or replace MAF sensor.
- Clear codes and road-test; re-scan after two drive cycles if the monitor must set.
Topics
- OBD-II and scan tools (guide)
- Powertrain DTCs (P-prefix) (system)
Related symptoms
- Check engine light (MIL) (weight 0.820)
- Rough idle (weight 0.780)
- Poor fuel economy (weight 0.620)
Common fixes (curated)
- Diagnose and repair vacuum / air leak (rank 1)
- Replace oxygen sensor(s) (rank 2)
- Clean or replace MAF sensor (rank 3)
Related articles
- Engine Vacuum Leak Causing Rough Idle & Lean Codes 2019 Toyota Prius Prime
- Engine Vacuum Leak: Rough Idle & Lean Codes on 2020 Highlander
- Engine Runs Lean with Check Engine Light 2017 Toyota Rav4
- Engine Running Lean Diagnosis 2018 Toyota Corolla Im
- Engine Running Lean Diagnosis 2019 Toyota 86
- Engine Running Lean with Check Engine Light 2020 Toyota Mirai
- Engine Running Lean Diagnosis 2019 Toyota Camry
- Engine Vacuum Leak Symptoms 2001 Toyota Corolla
- Engine Vacuum Leak Causing Hissing Sound 1999 Ford F-450
- Engine Vacuum Leak Symptoms 1999 Ford Crown Victoria
- Engine Running Lean Diagnosis: 1997 Ford At8513 Troubleshooting
- P0171 field guide: system too lean (Bank 1)
Diagnostic bundle (JSON)
Machine-readable graph: related codes, symptoms, fixes, topics, articles, and curated sources rows. Append &external=1 for optional YouTube / Google CSE hits when API keys are configured (see web/.env.example).