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 PCM can make corrections from a nominal 128 short/long term fuel trim value
(0 percent correction) within a maximum window of 70-165 (-45 percent
to +29 percent correction) without setting DTC P0171 or P0172. If the
PCM determines a rich condition exists, the O2S-1 voltage is 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 70-128. If the PCM determines a lean condition
exists, O2S-1 voltage is 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
128-165. When certain conditions have been met (ECT and closed loop), the PCM
will go into a leaned fuel control mode by updating and using a total of 22 long
term fuel trim cells for 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 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.
Conditions for Setting the DTC
DTC P0171 will set if the LTFT value is greater than 165, greater than 29 percent
correction, when:
| • | The condition exists for longer than 16 seconds. |
| • | The ECT is between 60-115°C (140-239°F). |
| • | The IAT is between -25° and +115°C (-13° and +239°F). |
| • | The MAP is greater than 28 kPa (auto) or 26 kPa (manual). |
| • | The engine speed is between 550-3,600 RPM (auto) or 750-3,600 RPM
(manual) |
| • | The vehicle speed is less than 132 km/h (82 mph). |
| • | The fuel level is greater than 9.8 percent. |
| • | The barometric pressure is greater than 72 kPa. |
| • | No CAM, CKP, ECT, fuel injector, IAT, knock, MAP, misfire, TP, O2S-1,
vehicle speed, P0446, P0601, P0602, or P1441 DTCs have been set |
DTC P0171 diagnostic runs continuously once the above conditions have been met.
DTC P0171 is a type B DTC.
Diagnostic Aids
DTC P0171 indicates the engine is running lean. The following are possible causes
of a lean air/fuel mixture:
| • | Fuel quality poor or water in fuel |
| • | Exhaust manifold gasket leak or leak around O2S-1 |
To verify the system is running lean, use the scan tool to monitor O2S-1 and
HO2S-2. If both sensors are below 300 mV, the system is running lean and the
oxygen sensors are probably not at fault.
