Code
P0171
Fuel / Air
System too lean — Bank 1
The engine runs with too much air or too little fuel on bank 1. Fuel trims add fuel to compensate.
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Most likely causes
- Vacuum leak (intake, PCV, brake booster)
- Dirty or faulty mass airflow sensor
- Clogged fuel filter or weak pump
- Faulty oxygen sensor
- Exhaust leak
Symptoms you may notice
- Check engine light
- Poor idle or stalling
- Hesitation under load
- Higher fuel consumption
Checks to run
- Smoke test the intake for leaks
- Clean and test the MAF sensor
- Check fuel pressure and filter
- Monitor fuel trims live
Tools you may need
OBD-II scanner
Smoke machine
Fuel pressure gauge
MAF cleaner
Always diagnose before replacing. Codes share symptoms; confirm the cause with measurements before buying parts.