P0172

The front oxygen sensor sends signals on exhaust gas oxygen content to the PCM to control a 14.7 to 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 70 to 165 (-46% to 32%). 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 to 1 air/fuel ratio. The short term fuel trim value will be between 70 (-46%) and 128 (0%) 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 (0%) and 165 (32%) 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 one of the long term cells being used.

DTC PARAMETERS
DTC P0172 will set if the fuel trim index is below 70 (-46%) for 16 seconds and:
^ Engine coolant is between 60° and 115°C (140° and 239°F).
^ Manifold pressure is above 26 kPa (4 psi) (manual) or 28 kPa (4 psi) (auto).
^ RPM is between 750-3600 (auto) or 850-3400 (manual).
^ Barometric pressure is greater than 72 kPa (10 psi).
^ IAT reading is between -25° and 115°C (-13°F and 239°F).
^ Fuel level is greater than 9.8%.
^ Vehicle speed is less than 132 km/h (82 mph).
^ No Engine Sensor, VSS, EVAP Canister Purge, Front O2S, Idle Speed, PCM Internal, Misfire, Fuel Injector or P0125 DTCs have been set.

IMPORTANT: The fuel control system is continuously checked when the above conditions have been met.

Possible Causes
The following are malfunctions that could cause a rich condition:
^ Fuel pressure too high. Perform fuel pressure test.
^ Leaking fuel injector(s). - Perform pressure leakdown test.
^ Vacuum leak around the MAP sensor grommet.
^ Restricted exhaust or air intake/filter.
^ Fuel Vapor Canister saturated with fuel. (Refer to "Fuel System".)
^ A shorted 5 volt reference circuit. Check 5 volt reference sensors for abnormal readings.
^ Loose or erratic TP sensor.
^ Oxygen sensor contamination.

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