What P0171 Means
The engine control module is adding more fuel than normal to compensate for a mixture that's running leaner (too much air, not enough fuel) than it should — and it's had to add more correction than it's allowed to.
Common Causes
- Vacuum leak (cracked hose, bad intake gasket)
- Failing mass airflow (MAF) sensor
- Weak fuel pump or low fuel pressure
- Clogged or dirty fuel injectors
- Faulty oxygen sensor giving an inaccurate reading
- Exhaust leak before the oxygen sensor
Symptoms You Might Notice
- Rough or high idle
- Hesitation or stumbling on acceleration
- Check engine light on
- Reduced fuel economy
- Occasional stalling
What to Check First
- 1
Inspect all vacuum hoses and intake gaskets for cracks or leaks — this is the most common cause
- 2
Check the MAF sensor for dirt or contamination, and clean or replace as needed
- 3
Test fuel pressure against the manufacturer's specification
- 4
Check for exhaust leaks upstream of the oxygen sensor
Frequently Asked Questions
What's the difference between P0171 and P0174?
P0171 is Bank 1, P0174 is Bank 2 — the same lean-mixture issue, just detected on a different bank of cylinders. On 4-cylinder engines with only one bank, you'll only ever see P0171.