Powertrain Management (CNG)

DTC P0121

Circuit Description

The throttle position (TP) sensor is a potentiometer. The powertrain control module (PCM) supplies about 5 volts on the TP sensor reference voltage circuit. The PCM also supplies a ground circuit to the TP sensor. When the throttle is depressed, the TP signal voltage rises to near the supplied 5 volts. As the throttle is released, the TP signal voltage drops to around 0.6 volts. The PCM and the Alternative Fuels ECM monitor the TP signal circuit voltage and determine the angle of the throttle blade.


This DTC sets when the throttle angle is greater than calculated, based upon engine speed and engine manifold absolute pressure (MAP).


Conditions for Running the DTC

* The engine is operating on alternative fuel.
* The engine has been operating for greater than 2 seconds.
* No Alternative Fuel ECM DTCs are set. DTCs P1215, P1432, and P1433 may be set.
* The engine MAP is less than 45 kPa.
* The TP sensor angle has changed less than 2 percent for 2 seconds continuously.



Conditions for Setting the DTC

The TP sensor angle is greater than the specified percentage at the given engine speed.






Action Taken When the DTC Sets




* The Alternative Fuels ECM will illuminate the MIL and store the DTC as History during 1 drive trip in which 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. The step verifies that a PCM specific MAP sensor condition is not the cause of the DTC. The MAP sensor display should change when the engine is started. The MAP value should change with the engine speed. If a MAP sensor voltage changes, there is no condition with the MAP sensor, or PCM wiring.
3. The step verifies that a PCM specific TP sensor condition is not the cause of the DTC. The TP sensor value should change with the engine speed. If the TP sensor voltage changes, there is no condition with the TP sensor.
4. The step verifies that a Alternative Fuels ECM specific MAP sensor condition is not the cause of the DTC. The MAP sensor display should change when the engine is started. The MAP value should change with the engine speed. If a MAP sensor voltage changes, there is no condition with the MAP sensor, or Alternative Fuels ECM wiring.
5. The step verifies that a specific TP sensor condition is not the cause of the DTC. The TP sensor value should change with the engine speed. If the TP sensor voltage changes, there is no condition with the TP sensor.
8. This step determines if the Alternative Fuels ECM or wiring is the short to voltage.
9. This step tests the 5 volt reference circuit, signal circuit, and the PCM. If the scan tool displays 5 volts, the TP sensor 5 volt reference and the TP sensor signal circuits are OK.
10. This step tests the TP sensor ground circuit. If the test lamp does not illuminate, test the ground circuit for an open or high resistance.