Circuit Description
The DTC P0172 Fuel Trim System Rich diagnostic monitors the heated oxygen sensor (HO2S) indications of the air/fuel mixture. 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 percent. 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 percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below 0 percent 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.
HO2S Indication | Short Term FT Action | Long Term FT Action | Short Term FT Response |
|---|---|---|---|
Lean Condition | Quick Increase of 1% or More | Slow Increase of 1% or More | Return to 0% |
Desired Condition | Stays Near 0% | Stays at Learned +/- Value | Stays Near 0% |
Rich Condition | Quick Decrease of -1% or Less | Slow Decrease of -1% or Less | Return to 0% |
Condition for Running the DTC
| • | The engine coolant temperature (ECT) is more than 70°C (158°F). |
| • | The engine is running at steady speed. |
| • | The engine is operating in Closed Loop. |
Conditions for Setting the DTC
The fuel trim value is constantly low during stable engine operation.
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) after 2 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 that is blocked by debris can cause a DTC P0172 to set. |
| • | Check for the correct fuel pressure. The fuel system will be rich if the 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 . |
| • | Check for fuel contamination of the crankcase. Fuel that has entered the crankcase can release vapors that enter the engine through the positive crankcase ventilation (PCV) system, thereby causing a rich running condition. |
| • | Check for a damaged wiring harness. If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving the connectors and the 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 the 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 mass air flow (MAF) sensor for a faulty electrical connection to the PCM. |
An intermittent malfunction may be caused by a fault in the HO2S 1 electrical circuit. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .
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 Diagnostic System Check-Engine Controls 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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Engine control sensors that are found to be out range (skewed) or fixed, can cause the engine to run rich. Inspect the performance of the MAF sensor. Inspect for air restrictions and air inlet leaks near the MAF sensor. If the HO2S voltage is fixed low, while the fuel trim is constantly high, check the MAF sensor circuits for resistance. High circuit resistance can cause the sensor to read the incorrect volume of air, resulting in the wrong fueling condition. Refer to the applicable sensor DTC table for direction in diagnosing any possible sensor error.
Step | Action | Values | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||||||||
Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | ||||||||
2 |
Is the Short Term FT parameter displayed on the scan tool more than the specified value? | -20% | Go to Diagnostic Aids | Go to Step 3 | ||||||
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? | -- | Go to Step 9 | Go to Step 4 | |||||||
Was a repair necessary? | -- | Go to Step 9 | Go to Step 5 | |||||||
5 |
Is the fuel pressure within the specified range? | 301-347 kPa (44-50 psi) | Go to Step 6 | Go to Fuel System Diagnosis | ||||||
6 |
Was a problem found? | -- | Go to Step 9 | Go to Step 7 | ||||||
7 |
Did the sensor require replacement? | -- | Go to Step 9 | Go to Step 8 | ||||||
8 | Is a rich condition still present? | -- | ||||||||
9 |
Does the DTC run and pass? | -- | Go to Step 10 | Go to Step 2 | ||||||
10 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |||||||
