For 1990-2009 cars

The oxygen sensor 1 (O2S-1) sends voltage signals that relate the difference in oxygen between the atmosphere and exhaust gas. Under normal driving conditions, except power enrichment mode, the PCM attempts to maintain a 14.7:1 air/fuel ratio. The powertrain control module (PCM) can make corrections from a nominal 0 percent short/long term fuel trim value within a maximum window of -21 percent to +15 percent correction without setting DTC P0171 or P0172. If the PCM determines a rich condition exists (O2S-1 voltage above 600 mV), it will decrease injector pulse width to maintain a 14.7:1 air/fuel ratio. The short term fuel trim value will be between -21 percent and 0 percent. If the PCM determines a lean condition exists (O2S-1 voltage below 300 mV), it will increase injector pulse width to maintain a 14.7:1 air/fuel ratio. The short term fuel trim value will be between 0-15 percent. When certain conditions have been met -- engine coolant temperature (ECT) and closed loop, the PCM will go into a learned fuel control mode by updating and using four long term fuel trim cells: idle, cruise, accel and decel. These cells update based on the short term fuel trim value. If the short term fuel trim value goes a certain number of counts in one direction, the long term fuel trim value will update in the same direction, in the cell that corresponds with the drive condition the vehicle was in. After approximately 2 minutes of being in each of the long term fuel trim cells, the PCM will enable the evaporative emission (EVAP) purge solenoid (pulling fuel from the EVAP canister) and a new set of long term fuel trim cells called purge long term fuel trim cells will be used. However, DTC P0171 is run before the purge cells are used and sets when the combined average of the long term fuel trim cells for idle and cruise is above the calibrated long term fuel trim value.

### DTC Parameters

DTC P0171 will set if the combined average of the idle and cruise long term fuel trim cells greater than 15.4 percent correction and the current short term fuel trim cell is greater than 0 percent when:

• | The condition exists for longer than 3 seconds.

• | The PCM is using non-purge fuel trim cells.

• | The engine speed is between 500-4000 RPM.

• | The loop status is closed.

• | The ECT is between 60-115°C (140-259°F).

• | The intake air temperature (IAT) is between -20° and +80°C (10-176°F).

• | The manifold absolute pressure (MAP) is between 30-90 kPa.

• | The throttle position (TP) angle is less than 75 percent.

• | The barometric pressure is greater than 70 kPa.

• | The calculated air flow is between 2-80 gm/s.

• | No camshaft (cam), crankshaft position (CKP), ECT, exhaust gas recirculation (EGR) flow, EVAP purge solenoid, idle air control (IAC) valve, IAT, MAP, misfire, O2S-1, PCM internal fault or TP sensor DTCs have been set

DTC P0171 diagnostic runs continuously once the above conditions have been met.

DTC P0171 is a type D DTC.

### Diagnostic Aids

DTC P0171 indicates the engine is running lean. The following are possible causes of a lean air/fuel mixture:

• | Vacuum leak

• | Fuel pressure low

• | Fuel quality poor or water in fuel

• | Exhaust manifold gasket leak or leak around O2S-1

• | Fuel injectors clogged

• | O2S-1 weak