Description
In order to provide the best possible combination of driveability, fuel economy, and emission control, the powertrain control module (PCM) uses a Closed Loop air/fuel metering system. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and when in Closed Loop adjusts fuel delivery based on the HO2S signal voltage. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on a scan tool. The ideal fuel trim value is around 0%. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above 0%. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below 0% indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition.
Conditions for Running the DTC
| • | Barometric pressure greater than 75 kPa. |
| • | Fuel tank level greater than 25%. |
| • | Intake air temperature is between -10°C to 70°C (14°F to 158°F). |
| • | Engine is running at a steady speed. |
| • | Engine is operating in Close Loop. |
Conditions for Setting the DTC
| • | Total fuel trim is lower than or equal to -30% for 5 seconds. |
| • | Short term fuel trim is lower than or equal to -20% for 45 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active. |
| • | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer. |
| • | The PCM enters the Fail-Safe Function and stops EGR control and EVAP purge control. |
Conditions for Clearing the MIL/DTC
| • | The MIL turns OFF after three consecutively passing trips without a fault present. |
| • | A History DTC clears after 40 consecutive warm-up cycles without a fault. |
| • | Use the scan tool Clear DTC Information function or disconnect the PCM battery feed in order to clear the DTC. |
Diagnostic Aids
Check for any of the following conditions:
| • | Check for a restriction in the inlet air passage. A duct or inlet hose that collapses when hot or is blocked by debris can cause a DTC P0172 to set. |
| • | Check for the correct fuel pressure. The fuel system will be rich if fuel pressure is too high. In order to determine the cause of a DTC P0171 monitoring the fuel pressure while driving the vehicle at various road speeds may be necessary. Refer to Fuel System Diagnosis . |
| • | A MAP sensor output that causes the PCM to sense a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the PCM to substitute a fixed (default) value for the MAP sensor. If the rich condition is gone when the MAP sensor is disconnected, substitute a known good MAP sensor and recheck. |
| • | Check for a damaged wiring 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 the connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault. |
| • | Check for a contaminated oxygen sensor. Silicone and other contaminants can cause the oxygen sensor to provide false exhaust oxygen content readings. If contamination is found, determine the cause and correct the condition before replacing the HO2S. |
| • | Inspect the HO2S 1 for water intrusion into the wiring harness and sensor housing. Water can create a short to voltage in the HO2S signal circuit causing a false rich indication. |
| • | Check the HO2S 1 and the MAP sensor for a faulty electrical connection to the PCM. |
If a DTC P0172 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.
Test Description
The numbers below refer to the step numbers in the Diagnostic Table.
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The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
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This step determines if the fault is present.
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This step checks for excess fuel from the fuel injector. The fault may be the fuel injector or the fuel injector electrical circuit.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic System Check? | -- | |||||||||
2 |
Is the scan tool display greater than the specified value? | -20% | ||||||||
Was the scan tool display greater than the specified value while operating the vehicle within the Freeze Frame conditions? | -30% | Go to Diagnostic Aids | ||||||||
4 |
Was a repair necessary? | -- | ||||||||
5 |
Is the fuel pressure within the specified value? | 90-140 kPa (13-20 psi) | Go to Fuel System Diagnosis | |||||||
6 |
Was a fault found and repaired? | -- | ||||||||
7 |
Important: Repair the cause of the contamination before replacing a contaminated HO2S. Did the HO2S 1 require replacement? | -- | ||||||||
Check the fuel injector for any of the following conditions:
Did the fuel injector pass all the checks? | 0.5 to 1.5 ohms at 20°C (68°F) | Go to Diagnostic Aids | ||||||||
9 | Repair/Replace the fuel injector. Refer to Fuel Injector Replacement . Is the action complete? | -- | -- | |||||||
10 |
Are any DTCs displayed on the scan tool? | -- | Go to the Applicable DTC Table | System OK |
