P0171

Documented for: 2.0L gasoline; 2.5L/3.0L gasoline · System(s): EMS EOBD, EMS OBD II · Factory MIL flag: 2 · Default action: ECM Default: –Bank 1 catalyst warm-up ignition retard inhibited –Bank 1 closed loop fuel metering inhibited –Canister purge inhibited –Maximum engine speed reduced

Common causes

• Engine misfire Air intake leak between MAF Sensor and throttle Fuel filter / system restriction Fuel injector restriction Low fuel pump output HO2 Sensor(s) (1/1, 1/2) harness wiring condition fault Exhaust leak (before catalyst) ECM receiving incorrect signal from one or more of the following components: ECT Sensor, MAF Sensor, IAT Sensor, TP Sensor
• Engine misfire Air intake leak between MAF Sensor and throttle Fuel filter / system restriction Fuel injector restriction Fuel pressure sensor failure (low fuel pressure) Low fuel pump output HO2 Sensor(s) (1/1, 1/2) harness wiring condition fault Exhaust leak (before catalyst) ECM receiving incorrect signal from one or more of the following components: ECT Sensor, MAF Sensor, IAT Sensor, IP Sensor, EFT Sensor, TP Sensor
• System Too Lean (Bank 1) - Vacuum leak
• low fuel pressure
• faulty MAF

Quick checks

• Confirm freeze-frame and conditions when the code set
• Inspect connectors, grounds, and harness for the circuit named in the code
• Compare live sensor values vs expected (unplugged/biased sensors are common)
• Check intake boots, PCV, and vacuum lines for splits (very common on aged X-Types)
• Swap coil/plug to another cylinder if misfire is cylinder-specific

Typical fixes

• Check for vacuum leaks
• test fuel pressure