Can I keep driving?
Usually safe to drive short-term, but a lean engine runs hotter and can misfire. Book a diagnosis soon.
What it means
The engine is getting more air than the computer expects, so it keeps adding fuel until it hits its limit. P0171 is bank 1, P0174 is bank 2 — both together usually means a shared cause like a vacuum leak or MAF sensor.
Common symptoms
- Rough or hunting idle
- Hesitation on acceleration
- Whistling or hissing from the engine bay
- Misfire codes may follow
Common causes
- Cracked intake boots or vacuum hoses
- Failed PCV / crankcase ventilation valve (very common on BMW)
- Dirty or failing mass airflow (MAF) sensor
- Leaking intake manifold gasket
- Low fuel pressure or weak fuel pump
- Exhaust leak ahead of the oxygen sensor
How we diagnose it
- Review short- and long-term fuel trims at idle and at speed
- Smoke test the intake for vacuum leaks
- Test the crankcase ventilation system
- Compare MAF readings to specification
- Measure fuel pressure
Where we see it most: BMW (PCV / crankcase vent valve), Mercedes-Benz, VW/Audi
A code points to the system that reported a problem, not the exact failed part. Replacing parts based on the code alone often wastes money — testing finds the real cause.