Lean & Rich Fuel Mixture Codes
Lean and rich fuel mixture codes indicate that the engine's air-to-fuel ratio is outside the optimal range. The ECU continuously adjusts fueling to maintain the ideal 14.7:1 air-to-fuel ratio — when it can no longer compensate, it sets a lean or rich code.
Last reviewed May 2026 · Find This Code Editorial Team
What "lean" and "rich" mean
Lean mixture (too much air)
More air than fuel in the combustion chamber. The ECU tries to add fuel via positive fuel trim corrections. When it reaches its limit (~25% correction), it sets a lean code.
Common causes: Vacuum leaks, dirty MAF sensor, weak fuel pump, clogged fuel filter, stuck-open injector.
Rich mixture (too much fuel)
More fuel than air in the combustion chamber. The ECU tries to reduce fuel via negative fuel trim corrections. When corrections reach their limit, it sets a rich code.
Common causes: Leaking fuel injectors, faulty coolant temp sensor, failing O2 sensor, failing MAF sensor reading low airflow.
Tip: Use a scan tool to monitor Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT). Values above +10% confirm a lean condition; below −10% confirm a rich condition. This data tells you whether the code is active and how severe the mixture deviation is.
P0171 alone vs. P0171 + P0174 together
P0171 alone (lean on Bank 1 only) suggests the problem is localized to the Bank 1 side of the engine — a Bank 1 vacuum leak, a faulty Bank 1 injector, or a Bank 1-specific component.
P0171 + P0174 together (lean on both banks) almost always points to a centralized cause that affects all cylinders equally: a large vacuum leak near the intake manifold, a dirty or failing MAF sensor, or low fuel pressure from a weak fuel pump.