Powertrain Management (CNG)

DTC P0172

Circuit Description

The heated oxygen sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean, the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 360°C (600°F) to produce voltage. The HO2S will produce more voltage when the exhaust is rich, and less voltage when the exhaust is lean. The HO2S operating voltage range is between 100 mV to 1,000 mV.


The Alternative Fuels ECM utilizes a Closed Loop air/fuel metering system when the HO2S is at operating temperature. During normal Closed Loop fuel control operation, the Alternative Fuels ECM will add fuel when the HO2S indicates a lean exhaust content. When the HO2S indicates a rich exhaust content, the Alternative Fuels ECM will subtract fuel. This addition and subtraction to the air/fuel mixture may be indicated on a scan tool as the Long and Short Term Fuel Trim values. Short Term Fuel Trim values are adjustments to the fuel mixture that occur quickly. Long Term Fuel Trim values are adjustments to the fuel mixture that accumulate over longer periods of time. The Long and Short Term Fuel Trim values are displayed on a scan tool as both percent and counts. Positive percentage values indicate that fuel is being added to the mixture. Negative percentage values indicate that fuel is being subtracted from the mixture.


This DTC is used to indicate a fully lean fuel trim system that cannot compensate for a rich engine exhaust content.


Conditions for Running the DTC

* The engine is operating on alternative fuel.
* The engine coolant temperature is more than 49°C (120°F).
* The Alternative Fuels ECM is operating in Closed Loop fuel control.



Conditions for Setting the DTC

The Alternative Fuels ECM monitors that the Short Term Fuel Trim is at -50 percent OR Long Term Fuel Trim is at -45 percent for at least 5 seconds continuously.


Action Taken When the DTC Sets




* A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
* The Alternative Fuels ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.
* The Alternative Fuels ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.



Conditions for Clearing the MIL/DTC




* The Last Test Failed status will clear when the DTC runs and passes.
* The Alternative Fuels ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
* The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
* All DTC Information can be cleared with a scan tool.



Diagnostic Aids

For an intermittent condition, refer toIntermittent Conditions .


Test Description

The numbers below refer to the step numbers on the diagnostic table.
2. This step determines whether the fault is present. More than values equal -46, -47.
4. If DTC P0172 and DTC P0175 set at the same time, then both banks of the engine are operating rich. Inspect items that would cause both banks to operate rich.