Circuit 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 the 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.
Condition for Running the DTC
| • | Engine coolant temperature is greater than 70°C (158°F). |
| • | Engine is running at steady speed. |
| • | Engine is operating in Closed Loop. |
Conditions for Setting the DTC
The fuel trim value is constantly low (negative percent) during stable engine operation.
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 stores the conditions which were present when the DTC set as Freeze Frame Data. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present. |
| • | A DTC will clear after 40 consecutive warm-up cycles have occurred without a fault. |
| • | A DTC can be cleared by using the scan tool Clear DTC Information function. |
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 P0172 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 O2S 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 O2S 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 O2S 1 and the MAP sensor for a faulty electrical connection to the PCM. |
An intermittent malfunction may be caused by a fault in the O2S 1 electrical circuit. Inspect the wiring harness and components for any of the following conditions:
| • | Backed out terminals. |
| • | Improper mating of terminals. |
| • | Broken electrical connectors locks. |
| • | Improperly formed or damaged terminals |
| • | Faulty terminal to wire connections. |
| • | Physical damage to the wiring harness. |
| • | A broken wire inside the insulation. |
| • | Corrosion of electrical connections, splices, or terminals. |
Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.
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 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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A plugged air filter can cause a rich condition and set a DTC P0172.
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If one of the engine control sensors are found to be out range (skewed) or stuck, refer to the applicable diagnostic table.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | |||||||||
2 |
Is the Short Term FT parameter displayed on the scan tool greater than the specified value? | -20% | Fault Not Present-Go to Diagnostic Aids | |||||||
Visually and physically check the air filter and the air induction assembly for any of the following conditions:
Did any of the above inspections isolate a condition requiring a repair? | -- | |||||||||
Was a repair necessary? | -- | |||||||||
5 |
Is the fuel pressure within the specified value? | 206-255 kPa (31-37 psi) | Go to Fuel System Diagnosis | |||||||
6 |
Was a problem found? | -- | ||||||||
7 |
Did the sensor require replacement? | -- | ||||||||
8 | Is a rich condition still present? | -- | Go to Symptoms | |||||||
9 |
Are any DTCs displayed on the scan tool? | -- | Go to the applicable DTC table | System OK |
