Circuit Description
The powertrain control module (PCM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The long term fuel trim values change in response to trends in the short term fuel trim. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. You can use a scan tool in order to monitor the short and long term fuel trim. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the PCM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the PCM is reducing the amount of fuel in order to compensate for a rich condition. If the PCM detects an excessively Rich or Lean condition, the PCM sets a diagnostic trouble code (DTC). The long term fuel trim diagnostic parameter is an average of several of the long term speed load learn cells which the PCM selects based on the engine speed and the engine load.
Conditions for Running the DTC
| • | No active mass air flow (MAF) DTCs |
| • | No active manifold absolute pressure (MAP) DTCs |
| • | No active intake air temperature (IAT) DTCs |
| • | No active engine coolant temperature (ECT) DTCs |
| • | No active throttle position (TP) DTCs |
| • | No active injector DTCs |
| • | No active misfire DTCs |
| • | No active knock sensor (KS) DTCs |
| • | No active crank sensor DTCs |
| • | No active ignition control DTCs |
| • | No active exhaust gas recirculation (EGR) DTCs |
| • | No active secondary air injection (AIR) DTCs |
| • | No active evaporative emission (EVAP) DTCs |
| • | No active vehicle speed (VS) DTCs |
| • | The ECT is between 50°C (122°F) and 115°C (239°F). |
| • | The barometric pressure is more than 74 kPa. |
| • | The MAF is between 5 g/s and 90 g/s or between 10 g/s and 40 g/s if the excessive purge test is running (purge duty cycle more than 20 percent). |
| • | The MAP is between 26 kPa and 90 kPa. |
| • | The IAT is between -20°C (-4°F) and +90°C (+194°F). |
| • | The engine speed is between 400 RPM and 3,000 RPM or more than 800 RPM if the excessive purge test is running. |
| • | The TP sensor angle is less than 90 percent. |
| • | The vehicle speed is less than 137 km/h (85 mph). |
Conditions for Setting the DTC
| • | The average long term fuel trim cell values are below a predetermined threshold. |
| • | All the above conditions are present for 40 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails, if the diagnostic fails under the same conditions such as load, RPM, temperature, etc. as the previous ignition cycle that the test ran and failed. |
| • | The first time the diagnostic fails, the PCM records the operating conditions in Failure Records. |
| • | The second consecutive ignition cycle that this diagnostic fails under the same conditions such as load, RPM, temperature, etc. as the previous ignition cycle that the test ran and failed, the PCM stores this information in Freeze Frame. The PCM copies any data previously stored in Freeze Frame and copies the data into the Failure Records. The PCM then overwrites the Freeze Frame. The only exception to this is if a misfire DTC was already recorded in Freeze Frame. In this case, the misfire data stays in Freeze Frame and the PCM updates the fuel trim data in the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns the MIL OFF after three consecutive ignition cycles that the diagnostic runs and does not fail within the same conditions that the DTC last failed. |
| Important: If the last failure was during a non-typical driving condition, the MIL may remain ON longer than the three ignition cycles. Review the Freeze Frame and Failure Records for the last failure conditions. |
| • | A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures. |
| • | A last test failed clears when the diagnostic runs and does not fail. |
| • | Use a scan tool in order to clear the MIL/DTC. |
Diagnostic Aids
For an intermittent condition, refer to Symptoms .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This test determines whether the fault is present. More than values equal -12, -11.
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Using the Freeze Frame and Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass or a fail. Operate the vehicle within the same Freeze Frame conditions such as RPM, load, vehicle speed, temperature etc. that you observed. This will isolate when the DTC failed.
-
If DTC P0172 is also set, then both banks are operating rich. Inspect items that would cause both banks to operate rich.
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Excessive fuel in the oil will cause DTCs P0172 and P0175 to set at the same time. Remove the PCV valve from the intake manifold and plug with a suitable stopper. Also disconnect the fresh air pipe from the rocker cover and plug with a suitable stopper. If the long term and short term fuel trim values increase, this indicates there is fuel in the oil.
Step | Action | Value(s) | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||||||||
|
Important: If any DTCs are set, except P0172 and P0175, refer to those DTCs before proceeding with this diagnostic.
Is the LT FT more than the specified values indicated? | -13% LT | |||||||||||||
Does the scan tool indicate that this diagnostic failed this ignition? | -- | Go to Diagnostic Aids | ||||||||||||
Is DTC P0172 also set? | -- | |||||||||||||
Visually and physically inspect the following items:
Was a problem found in any of the above areas? | -- | Go to Fuel System Diagnosis | ||||||||||||
6 |
Important: When the fuel system test is finished, return to this table. Inspect the bank 2 injectors for leaking. Refer to Fuel System Diagnosis . Are any injectors leaking? | -- | ||||||||||||
7 | Repair or replace any faulty items found. Is the action complete? | -- | -- | |||||||||||
8 | Replace the faulty HO2S. Refer to Heated Oxygen Sensor Replacement - Bank 2 . Is the action complete? | -- | -- | |||||||||||
9 |
Does the scan tool indicate that this test ran and passed? | -- | ||||||||||||
10 | Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to the applicable DTC table | System OK |
