Description
In order to provide the best possible combination of driveability, fuel economy, and emission control, the engine control module (ECM) uses a Closed Loop air/fuel metering system. The ECM 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 ECM 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 ECM 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 Setting the DTC
| • | Total fuel trim less than or equal to -22% in 128 firing events. |
| • | Short term fuel trim less than or equal to -11% in 128 firing events. |
| • | Barometric pressure less than 75 kPa. |
Action Taken When the DTC Sets
| • | The ECM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active. |
| • | The ECM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer. |
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 ECM battery feed in order to clear the DTC. |
Diagnostic Aids
Check for any of the following conditions:
| • | If any DTCs other than Fuel Trim P0171, P0172, Misfire P0300 to P0304 and EGR P0400 are set, diagnose those DTCs first. |
| • | 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 malfunctioning MAF sensor. |
| • | 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 MAF sensor for a faulty electrical connection to the ECM. |
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 checks whether the fault is still present.
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This step checks for a Total Fuel Trim value that is a negative number that is numerically higher than -22%, such as -23% or -24%. Total Fuel Trim is a combined value of Short Term FT and Long Term FT.
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If the condition is still present, any one of these sensors could cause this DTC to set. Check the performance of the MAF sensor, the ECT sensor, the TP sensor and the CTP switch signal. Repair any faulty conditions.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | |||
Is the scan tool display greater than the specified value? | -11% | |||
Was the scan tool display greater than the specified value while operating vehicle within the Freeze Frame conditions? | -22% | Go to Diagnostic Aids | ||
4 | Visually and physically inspect the air filter and the air filter assembly for restrictions. Repair or replace as necessary. Was a repair necessary? | -- | ||
5 |
Is the fuel pressure within the specified value? | 210-260 kPa (30-37 psi) | Go to Fuel System Diagnosis | |
6 |
Was a problem found? | -- | ||
7 |
Important: Before replacing the oxygen sensor because of contamination, the cause of the contamination must be determined and corrected.
Did the HO2S 1 require replacement? | -- | Go to Symptoms | |
Are any DTCs displayed on the scan tool? | -- | Go to the Applicable DTC Table | System OK |
