For 1990-2009 cars

The front oxygen sensor sends signals on exhaust gas oxygen content to the PCM to control a 14.7:1 air/fuel ratio under normal driving conditions. The PCM can make fuel corrections from a nominal 128 (0% correction) short/long term fuel trim value to a maximum window of 101-147 (-21 to +15 percent). If the PCM determines a rich condition exists, 02 voltage above 450 mV, it will decrease injector pulse width, reducing fuel, to maintain a 14.7:1 air/fuel ratio. The short term fuel trim value will be between 101-128 indicating a rich condition. If the PCM determines a lean condition exists, 02 voltage below 450 mV, it will increase injector pulse width, adding fuel, maintaining a correct air/fuel mixture. The short term fuel trim value will be between 128-147 indicating a lean condition. When certain conditions have been met to begin learning fuel control (ECT, closed loop, etc.) the PCM will use the short term fuel trim cell to update 1 of the 4 long term cells (idle, decel, cruise, accel) being used. During every drive cycle, or trip, the PCM will use and update the idle and cruise long term cells and will keep track of how long the control system is operating in these cells. After approximately 2 minutes of being in each of these cells, the fuel control system will enable EVAP purge and the purge long term cells will be used. Refer to EVAP Canister Purge for information on purge long term fuel cells. If the average of the idle and cruise fuel cells is 147 and the short term fuel trim cell is currently over 128, a lean fuel system DTC will set. This test is performed before the purge cells are used.

### Conditions for Setting the DTC

Important: The fuel control system is continuously checked when the following conditions have been met.

DTC P0172 will set if the average long term idle and cruise fuel trim cells are 101 and the current short term fuel trim cell is below 128 for 3 seconds when:

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

• | The system is in closed loop.

• | The engine coolant is between 60-115°C (140-239°F).

• | Air flow is between 2-80 grams/second.

• | Manifold pressure is between 20-90 kPa (3-13 psi).

• | The RPM is between 500-4,000 RPM.

• | The TP is less than 75 percent open.

• | The barometer is greater than 70 kPa (10 psi).

• | The IAT reading is between -30 to +80°C (-22 to +176°F).

• | The vehicle speed is less than 121 km/h (75 mph).

• | No ECT, MAP, IAT, TP sensor, VSS, EVAP canister purge, front O2S, or P0125 DTCs have been set.

#### Possible Causes

Important: If the front oxygen reading voltage is above 600 mV and the rear HO2S-2 is above 600 mV all the time, the O2 sensors are probably not at fault.

The following are malfunctions that could cause a rich condition:

• | Fuel pressure too high -- Perform the fuel pressure test. Refer to Engine/Fuel/Air Intake/Exhaust Service.

• | Leaking fuel injectors -- Perform the pressure leakdown test. Refer to Engine/Fuel/Air Intake/Exhaust.

• | Vacuum leak around the MAP sensor grommet

• | Oxygen sensor contamination

• | Restricted exhaust or air intake

• | Fuel vapor canister saturated with fuel -- Refer to Engine/Fuel/Air Intake/Exhaust.