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 powertrain control module (PCM) attempts to maintain
a 14.7:1 air/fuel ratio. The 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 (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 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 -21.1 percent correction and the current short term fuel
trim cell is less 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 engine coolant temperature (ECT) is between 60-115°C (140-259°F). |
| • | 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 cam, crankshaft position (CKP), ECT, exhaust gas recirculation (EGR)
flow, EVAP purge solenoid, idle air control (IAC) valve, IAT, MAP, O2S-1, TP or vehicle
speed DTCs have been set. |
DTC P0172 diagnostic runs continuously once the above conditions have been met.
DTC P0172 is a type D DTC.
Diagnostic Aids
DTC P0172 indicates the engine is running rich. The following are possible causes
of a rich air/fuel mixture:
| • | Vacuum leak around MAP sensor |
| • | EVAP purge solenoid stuck open |
| • | Restricted exhaust or restricted air induction |
