P0121

Diagnostic Chart:

Wiring Diagram:

Circuit Description

The PCM supplies a 5V reference signal and a ground to the Throttle position (TP) sensor. The TP sensor sends a voltage signal back to the PCM relative to the throttle plate opening. The voltage signal will vary from approximately 0.45V at closed throttle, to over 4.5V at wide open throttle. The PCM 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 verifies that the engine is stable and at idle, and ensures that the TP signal is below a given percent.

Conditions for Setting the DTC

DTC(s) P0107, P0108, P0122, P0123 not set.

Idle Test

^ Vehicle speed is less than 1 mph.

^ Engine speed is less than 850 RPM (Auto).

^ Engine speed is less than 1200 RPM (Man).

^ IAC counts are less than 145 counts.

^ Manifold Absolute Pressure (MAP) is stable.

And

TP angle is greater than 7% at idle after attempting to auto-zero.

Cruise Test

^ TP is stable ±2%.

^ MAP is less than 37 kPa.

And

TP angle is greater than the following specified percentages at the given RPMs.

^ TP angle is greater than 60% at 1600 RPM

^ TP angle is greater than 77% at 2400 RPM

^ TP angle is greater than 89% at 3200 RPM

^ TP angle is greater than 99% at 4000 RPM

Action Taken When the DTC Sets

^ The Malfunction Indicator Lamp (MIL) will illuminate.

^ The PCM 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.

^ 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 by using the scan tool.

Diagnostic Aids

^ A TP sensor that is stuck will attempt 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 V and is possibly stuck.

^ If a 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 be set while attempting to start a vehicle that has run out of fuel.

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

1. The Powertrain OBD 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 fault occurred. The information is then stored in 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 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 0.50V. 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 may be 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 more than 0.5V.

10. Disconnecting the EGR valve and sensors connected to the 5V reference circuits internally to the PCM determines if the 5V reference circuit is shorted or if one of the sensors is causing a short on the 5V reference circuit.

15. Replacement PCMs must be reprogrammed. Refer to the latest Techline information for programming procedures.

17. If no faults have been found at this point and no additional DTCs were set, refer to Diagnostic Aids for additional checks and information.