You're getting P0171: System Too Lean, Bank 1. Maybe with a P0174 alongside (Bank 2 — same problem on the other side of a V engine). Your engine is getting more air than it should relative to fuel. Here's how to find the leak.
What lean actually means
The ideal air/fuel ratio is 14.7 parts air to 1 part fuel by mass (stoichiometric). The ECU measures air coming in via the MAF sensor and squirts fuel to match. When the O2 sensors see the exhaust and calculate that combustion happened lean, they tell the ECU to add more fuel next cycle — that's the "short-term fuel trim" (STFT) going positive.
If the ECU has to add more than ~10% extra fuel consistently, "long-term fuel trim" (LTFT) also goes positive and stays there. When LTFT crosses ~25% (varies by manufacturer), P0171 sets.
The 3 causes ranked by likelihood
#1: Vacuum leak (60% of cases)
A crack, torn hose, or bad gasket lets air INTO the intake AFTER the MAF sensor measures it. The engine gets that air but the ECU doesn't know about it, so it doesn't add fuel to compensate. Lean.
How to find:
- Start with the visible: PCV valve and hose, intake manifold vacuum ports, brake booster hose, EGR valve gasket, throttle body gasket, intake manifold gasket. Check for cracks, tears, disconnected hoses.
- Do the "propane test" or "carb cleaner test": with engine at warm idle, spray small amounts around suspect areas. If idle RPM changes when you spray a spot, you found the leak.
- For deep-buried leaks: get a shop to smoke-test the intake. $50-100. They pressurize the intake with visible smoke, and every leak shows itself.
#2: Dirty MAF sensor (25% of cases)
The MAF sensor is a heated wire that measures airflow by how much the wire cools. Dirt or oil on the wire insulates it — the ECU thinks less air is flowing than actually is, so it adds less fuel. Lean.
Common culprit: aftermarket oiled air filters (K&N etc.) that dump oil onto the MAF over time.
Fix ($8, 5 min): buy MAF-specific cleaner (CRC MAF Sensor Cleaner). Do NOT use regular carb cleaner — it'll damage the sensor. Remove the MAF, spray, let dry completely, reinstall. Costs less than a coffee and fixes lean codes in a huge percentage of cases.
#3: Fuel delivery (15% of cases)
The engine wants more fuel and the fuel system can't deliver:
- Weak fuel pump — pressure test. Should be 45-60 psi at idle (check your manual). Below spec = failing pump. $150-300 part, 2-4 hr DIY.
- Clogged fuel filter — if your car has a serviceable filter (most modern cars have it in the tank now), replace. $15-30 part.
- Bad fuel pressure regulator — check the vacuum line to the regulator for fuel. If fuel comes out, the diaphragm is torn. $30-80 replacement.
- Clogged injectors — try fuel system cleaner ($15). If not resolved, ultrasonic cleaning at a shop (~$100).
What about the O2 sensor?
An O2 sensor CAN cause false lean codes if it's biased low. But it's rare (~5% of cases) and easy to test: look at both O2 sensors in live data. If Bank 1 upstream is reading way lower than Bank 2 upstream, sensor drift is possible.
Related codes
P0174— System Too Lean, Bank 2 (V engines). P0171 + P0174 together = definitely a big vacuum leak or MAF issue affecting both banks.P0172/P0175— the OPPOSITE (running rich). Usually leaky injector or bad fuel pressure regulator.P0300— misfire, often caused by the lean condition. Fix the lean, misfire clears.P0420— catalyst efficiency low. Chronic lean conditions can trigger this too.
How to check your progress
After each fix attempt, clear the code and drive. Watch Short-Term and Long-Term Fuel Trims in the Live Data screen (filter to Fuel category in the DIY Fix My Car app). Both should be close to zero. If LTFT drops from +25% to +5%, you're on the right track — the ECU is learning it doesn't need to add as much fuel.
Bottom line
Spend $8 on MAF cleaner and 30 minutes with carb cleaner around your intake before you spend anything else. 85% of P0171 cases are cured by one of those two. If you're still lean after that, move on to fuel delivery testing.