P0171Fuel Trim System Lean (Bank 1)
Get it checked soon
What it means
The oxygen sensors on bank 1 keep reporting more oxygen in the exhaust than expected, so the computer keeps adding fuel to compensate. When that correction — the fuel trim — climbs past its limit, P0171 sets. It means unmetered air is getting in, the airflow sensor is under-reporting, or fuel delivery is falling short.
Is it safe to drive?
Get it checked soon
Fine to drive in the short term, but a lean engine runs hotter and can start to hesitate, idle rough, or misfire — and prolonged hard driving on a badly lean mixture is how engines get damaged. Sort it before a road trip, not after.
Common causes
Most likely first.
- A vacuum leak — a cracked hose, leaking intake gasket, or bad PCV valve or hose
- A dirty MAF sensor under-reading the air coming in
- Weak fuel delivery — a clogged filter, tired pump, or failing regulator
- A leaking or torn intake boot between the MAF and the throttle
- A stuck-open EVAP purge valve leaning out the idle
- An exhaust leak near the upstream O2 sensor making the mixture read lean
What the live data shows
Long-term fuel trim on bank 1 is the headline: healthy is near 0%, and P0171 usually sets with LTFT holding high positive — +15% and beyond. Where it's worst is the clue: trims that improve as RPM rises point to a vacuum leak (a fixed-size leak matters most at idle); trims that worsen under load point to fuel delivery or the MAF. Cross-check MAF grams-per-second against RPM, and watch the bank 1 upstream O2 sensor hanging low.
A scanner that reads all 139 standard sensors shows you these readings live. ScanMyCar charts them as you drive, keeps the freeze-frame stored with the code, and pulls codes from every module — free, with any cheap Bluetooth adapter.
Typical fixes
Cheapest first, where honesty allows.
- Clean the MAF sensor with MAF cleaner — minutes and a few dollars
- Inspect and replace cracked vacuum and PCV hoses; smoke-test if nothing is obvious
- Replace the intake gasket if the leak traces there
- Test fuel pressure; replace the filter, regulator, or pump as indicated
A guide, not a diagnosis. This page describes what P0171 generally means across OBD-II cars. What's actually wrong with your car depends on its live data and its history — and any repair decision deserves a proper diagnosis, whether that's you with a scanner or a professional with a lift.