Circuit Description
To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. While in closed loop, the PCM monitors the HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0% if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, indicating that the PCM is reducing the amount of fuel delivered. If an excessively lean condition is detected, the PCM will set DTC P0171.
The PCMs maximum authority to control long term fuel trim allows a range between -23% and +16%. The PCMs maximum authority to control short term fuel trim allows a range between -11% and +20%. The PCM monitors fuel trim under various operating conditions (fuel trim cells) before determining the status of the fuel trim diagnostic. The fuel trim cells are weighted (assigned a percentage of weight in determining total fuel trim status for the diagnostic) as follows:
| • | Idle, purge commanded (Cell 0) |
| • | Deceleration, purge commanded (Cell 1) |
| • | Normal driving, purge commanded (Cell 2) |
| • | Deceleration, no purge commanded (Cell 6) |
| • | Normal driving, no purge commanded (Cell 7) |
The vehicle may have to be operated in all of the above conditions before the PCM determines fuel trim status. The amount that fuel trim deviates from 0% in each cell and the amount that each cell is weighted directly affects whether or not the vehicle must be operated in all of the cells described above to complete the diagnostic.
Conditions for Setting the DTC
| • | No MAF sensor, ECT sensor, HO2S, TP sensor, EVAP, EGR, Misfire, Injector circuit, Idle speed, MAP sensor, VSS, CMP sensor, CKP sensor, IC circuit or IAT sensor DTC(s) set. |
| • | Engine coolant temperature between 20°C (68°F) and 110°C (230°F). |
| • | Intake air temperature is between -18°C (0°F) and 65°C (149°F). |
| • | Manifold absolute pressure is between 18 kPa and 80 kPa. |
| • | Throttle angle is steady below 90%. |
| • | Vehicle speed is below 75 m.p.h. |
| • | Engine speed between 550 and 5000 RPM. |
| • | Long term fuel trim is at or near maximum authority of 16%. |
| • | Short term fuel trim is at or near maximum authority of 20%. |
| • | Air flow between 3.6 and 175 gps. |
| • | BARO is greater than 70 kPa. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed. |
| • | The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using the scan tool. |
Diagnostic Aids
Check for the following conditions:
| • | Poor connection at PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection |
| • | Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving connectors and wiring harnesses related to the engine harness. A change in the display will indicate the location of the fault. |
Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
Test Description
The numbers below refer to the step numbers on the Diagnostic Table.
Step | Action | Value(s) | Yes | No | ||
|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic System Check performed? | -- | ||||
Are any DTCs set other than P0171? | -- | Go to the applicable DTC charts and repair the other DTCs before proceeding with this chart | ||||
3 |
Are the displayed values at or near the specified values?
S.T. FUEL TRIM is the second specified value. | +16% +20% | ||||
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Go to Diagnostic Aids | ||||
5 | Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections and repair any problem found. Did your inspection reveal a problem requiring repair? | -- | ||||
6 | Visually and physically inspect the crankcase ventilation valve for proper installation and repair any problem found. Refer to Positive Crankcase Ventilation Valve . Did your inspection reveal a problem requiring repair? | -- | ||||
7 |
Did your inspection of the throttle body reveal a condition requiring repair? | -- | ||||
8 | Start the engine. Observe the idle quality. Is a high or unsteady idle being experienced? | -- | ||||
9 | With the engine idling, observe the IDLE AIR CONTROL display on the scan tool. Is the displayed value greater than the specified value? (Value is displayed in counts). | 5 | ||||
10 |
Did your inspection reveal a vacuum leak? | -- | ||||
11 |
Did the System Check isolate a problem requiring repair? | -- | ||||
12 |
Was the fuel contaminated? | -- | ||||
13 |
Did your inspection reveal a condition requiring repair? | -- | ||||
14 |
Does S.T. FUEL TRIM value decrease to near the specified value? | 0% | ||||
15 | Perform the procedure in Fuel System Pressure Test. Refer to Fuel System Pressure Test . Repair fuel system problem if necessary. Did the Fuel System Pressure Test isolate a condition requiring repair? | -- | ||||
16 | Perform the Evaporative Emissions Control (EVAP) Canister Purge Valve System Check and repair EVAP system problem if necessary. Did EVAP Canister Purge Valve System Check isolate a problem? | -- | ||||
17 |
Did your inspection reveal a problem? | -- | ||||
18 | Visually and physically inspect the exhaust manifold for leaks and loose or missing hardware and correct any problem found. Did your inspection reveal a problem? | -- | ||||
Perform the Injector Balance Test and correct any problem found. Refer to Fuel Injector Balance Test . Did the Injector Balance Test isolate a problem? | -- | |||||
20 |
Did your inspection reveal a problem? | -- | Go to Diagnostic Aids | |||
21 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is action complete? | -- | -- | |||
22 |
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Repair complete |
