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) P0106, P0107, P0108, P0171, P0172, P0200, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341, P0342, P0404, P0405, P0440, P0442, P0452, P0453, P0502, P0506, P0507, P0601, P0602, P1441 not set.

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

^ Engine run time is greater than 20 seconds.

^ Engine Coolant Temperature (ECT) is greater than 20°C (68°F).

^ TP is stable + 2%.

^ MAP is less than 30 kPa. (Manual)

^ MAP is less than 45 kPa. (Automatic)

And

TP angle is greater than the following specified percentages at the given Rpms for at least 6 seconds.

^ TP angle is greater than 2% at 0 RPM.

^ TP angle is greater than 10% at 800 RPM.

^ TP angle is greater than 20% at 1600 RPM.

^ TP angle is greater than 25% at 2400 RPM.

^ TP angle is greater than 30% at 3200 RPM.

^ TP angle is greater than 35% at 4000 RPM.

^ TP angle is greater than 35% at 4800 RPM.

^ TP angle is greater than 40% at 5600 RPM.

^ TP angle is greater than 40% at 6400 RPM.

ACTION TAKEN WHEN THE DTC SETS

^ The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the malfunction present.

^ The PCM will record the operating conditions at the time that the diagnostic fails. This information will store in the Freeze Frame and Failure Records buffers.

^ A history DTC stores.

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.

^ The MIL/DTC(s) can be cleared by using the scan tool.

DIAGNOSTIC AIDS

^ A TP sensor that is stuck will attempt an auto zero every time that the engine runs. The ATP sensor will not display as high a percentage as the sensor normally when the sensor has auto zeroed at a higher than normal voltage.

^ A normal TP sensor that displays 0 percent will not display higher than 1 V and is possibility 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 in order to determine the status of the DTC.

^ A DTC P0121 may 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 malfunction 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 malfunctioning

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 and the crankshaft position system variation procedure must be performed. Refer to the latest Techline information for PCM reprogramming and also refer to the Crankshaft Position (CKP) System Variation Learn Procedure for the Crankshaft Position System Variation Procedure.

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.