P1171 — System Too Lean Banks 1 And 2 (Lean Fuel Fault)

Causes

• Fuel pressure too low (weak pump, clogged filter, faulty regulator)
• Large intake vacuum/leak downstream of MAF or upstream of both banks (intake manifold gasket, vacuum hoses, PCV)
• Faulty or contaminated MAF/MAP sensor giving low airflow reading
• Exhaust leaks upstream of oxygen sensors (affecting O2 readings)
• Faulty fuel pressure sensor or fuel rail sensor
• Faulty or slow O2/UEGO sensors or heater circuits (wrongly biased lean)

Symptoms

• Check Engine Light (MIL) on and P1171 stored
• Engine runs lean, hesitation, reduced power or poor acceleration
• Rough idle or surging under load
• Hard starting or stumble, especially under load
• Reduced fuel economy may be observed

What to check

• Scan for stored codes and freeze-frame/live data (fuel trims, O2 voltages, MAF, fuel pressure PIDs)
• Compare long-term and short-term fuel trims for both banks (large positive trims indicate lean)
• Check upstream O2 sensor voltages — should switch and show ~0.1–0.9V under normal conditions
• Fuel rail pressure test at key on/run and under load with mechanical pressure gauge or factory PIDs
• Inspect for vacuum leaks (visual, smoke test, listen for hiss)
• Inspect and test MAF and MAP sensors (voltage/gram/sec outputs)

Signal parameters

• Upstream O2 voltage: abnormally low or stuck near 0.1–0.3 V (lean)
• Short-term fuel trim (STFT): large positive values (e.g. +10% to +35%)
• Long-term fuel trim (LTFT): sustained positive correction (often >+10% is significant)
• MAF output (g/s or V): lower than expected for RPM/load
• Fuel rail pressure: below factory spec (typically >10–15 psi below spec is significant)
• MAP/vacuum: higher vacuum or abnormal MAP reading inconsistent with throttle position

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