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P0172

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 to 1 air/fuel ratio. The PCM can make corrections from a nominal 128 short/long term fuel trim value (0% correction) within a maximum window of 101 to 148 (-21% to 15% 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 to 1 air/fuel ratio. The short term fuel trim value will be between 101 and 128. 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 to 1 air/fuel ratio. The short term fuel trim value will be between 128 and 148. When certain conditions have been met (ECT and closed loop), the PCM will go into a leaned 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 two minutes of being in each of the long term fuel trim cells, the PCM will enable the 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 P0172 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 below the calibrated long term fuel trim value.

DTC PARAMETERS
DTC P0172 will set if the average idle and cruise long term fuel trim cells is less than or equal to 101 (greater than -21.1% correction) and the current short term fuel trim cell is less than 128 when:
^ Condition exists for longer than 3 seconds
^ PCM is using non - purge fuel trim cells
^ Engine speed is between 500 and 4000 RPM
^ Loop status is closed
^ ECT is between 60° and 115°C (140° and 259°F)
^ IAT is between -20° and 80°C (10° and 176°F)
^ MAP is between 30 kPa and 90 kPa
^ TP angle is less than 75%
^ Barometric pressure is greater than 70 kPa
^ Calculated air flow is between 2 and 80 gm/s
^ No cam, CKP, ECT, EGR flow, EVAP purge solenoid, IAC valve, IAT, MAP, misfire, O2S-1, TP or VSS DTCs have been set

DTC P0172 diagnostic runs continuously once the above conditions have been met.

DTC P0172 is a type A DTC.

DIAGNOSTIC AIDS
DTC P0172 indicates the engine is running rich. The following are possible causes of a rich air/fuel mixture:
^ Fuel pressure high
^ Fuel injector(s) leaking
^ O2S-1 contaminated
^ Vacuum leak around MAP sensor
^ EGR valve stuck open
^ EVAP purge solenoid stuck open
^ Restricted exhaust or restricted air induction

To verify the system is running rich, use Scan tool to monitor O2S-1 and HO2S-2. If both sensors are above 600 mV, the system is running rich and the oxygen sensors are probably not at fault.