P0121
DTC P0121 Throttle Position Sensor Rationality
Circuit Description
The powertrain control module (PCM)/Engine control module (ECM) supplies a 5 volt reference signal and a ground to the Throttle Position (TP) sensor. The TP sensor sends a voltage signal back to the PCM/ECM relative to the throttle plate opening. The voltage signal will vary from approximately 0.45 volts at closed throttle, to over 4.5 volts at wide open throttle (WOT). The PCM/ ECM can detect if the TP sensor is out of range while cruising by waiting for a Manifold Air Pressure (MAP) change and then checks if the TP sensor has also changed as expected by calculating what the TP signal should be at a given rpm and engine load. During idle conditions, the PCM/ECM verifies that the engine is stable and at idle, and ensures that the TP signal is below a given percent. The TP sensor is considered stuck when the MAP is low (<35 kpa) while the TP sensor position is greater than a certain value which will cause MAP to exceed the low.
Conditions for Setting the DTC
- DTC(s) P0107, P0108, P0117, P0118, P0122, P0123, P0201, P0202, P0203, P0204, P0300, P0351, P0352, P0402, P0404, P0405, P0406, P0443, P0506, P0507, P1404, P1441 and P1627 are not set.
- Engine running.
- The change in MAP is less than 2 kPa for greater or equal to 1 second prior to first enable.
- The IAC is between 10 and 160 cfs.
- The coolant temperature is greater than -10 °C.
Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The PCM/ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.
- A history DTC is stored.
- The coolant fan turns ON.
Conditions for Clearing the MIL/DTC
- The MIL will turn off after three consecutive ignition cycles in which the diagnostic runs without a fault.
- A history DTC will clear after 40 consecutive warm up cycles without a fault.
- DTC(s) can be cleared using the scan tool.
Diagnostic Aids
A TP sensor that is stuck will attempt to auto zero itself every time the engine runs. It will not display as high a percentage as it would normally when it has auto zeroed at a higher than normal voltage.
A normal TP sensor that displays 0% will not display higher than 1 volt and is possibly stuck.
If DTC P0121 cannot be duplicated, the information included in the Freeze Frame/Failure Records data can be useful. Use the scan tool DTC information data to determine the status of the DTC.
A DTC P0121 may set while attempting to start a vehicle that has run out of fuel.
Steps 1 - 5:
Steps 6 - 15:
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. The On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
2. The TP sensor has an auto zeroing feature. If the voltage reading is between 0.2 to 0.9 volts, the PCM/ECM will assume the TP sensor is at a closed throttle position (0%). If the reading is out of the auto zeroing range at closed throttle, look for a binding or damaged linkage.
3. Normal TP voltage when the throttle plates are fully closed is near 0.5 volts. A sensor will display a higher voltage when the sensor is stuck or a circuit is faulty.
5. A disconnected TP sensor should not display a voltage reading on the scan tool. An amount less than the specified value is normal.
6. When the test light is connected to the ground circuit, the light should glow brightly if the ground circuit is OK but could have a small amount of resistance in the circuit. The voltmeter is used to detect the small amounts of resistance using the voltage drop method. If the test light glows dimly or not at all then resistance in the circuit is present and using the voltmeter is not necessary.
7. In order for DTC P0121 to set with a short voltage in the TP signal circuit, the voltage must be minimal enough to not set a DTC P0123 and must be more than 0.5 volts.
13. The replacement PCM/ECM must be reprogrammed. Refer to the latest Techline procedure for PCM/ECM reprogramming.
15. If no faults have been found at this point and no additional DTCs were set, refer to "Diagnostic Aids" for additional checks and information.