Powertrain Management (CNG)

DTC P0108

Circuit Description

The manifold absolute pressure (MAP) sensor is a pressure sensor. The powertrain control module (PCM) supplies about 5 volts on the MAP sensor reference voltage circuit. The PCM also supplies a ground circuit to the MAP sensor. When the manifold vacuum is low, the MAP signal voltage rises to near 4.5 volts. As the manifold vacuum increases, the MAP signal voltage drops to around 1 volt. The PCM and the Alternative Fuel ECM monitor the MAP signal circuit voltage and determine the vacuum within the intake manifold.


Certain vehicle models will also use the MAP sensor in order to determine the barometric pressure (BARO) when the ignition switch is turned ON and the engine is NOT cranked. This BARO reading may also be updated whenever the engine is operated at wide open throttle.


This DTC is used to indicate a MAP sensor signal voltage that is above the normal operating range of the sensor.


Conditions for Running the DTC

* The engine is operating on alternative fuel.
* The engine speed is greater than 500 RPM.
* DTCs P0121, P0122, or P0123 are not set.
* The TP angle is less than 4 percent.



Conditions for Setting the DTC

The Alternative Fuel ECM monitors a MAP sensor signal voltage of greater than 4.8 volts for at least 4 seconds continuously.


Action Taken When the DTC Sets

* The Alternative Fuel ECM will switch-over to gasoline fuel operation.
* The Alternative Fuel ECM will not attempt alternative fuel operation until the next key cycle.



* 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.



Test Description

The numbers below refer to the step numbers on the diagnostic table.
2. This step determines if the condition is present.
3. This step determines if the condition affects the PCM.
4. This step determines if the MAP sensor is the shorted condition.
5. This step determines if an open sensor ground condition is present.
6. This step determines if the 5-volt reference circuit is shorted to a voltage. 5.2 volts is the maximum output that the PCM should supply on the 5-volt reference circuit.