Circuit Description
The throttle position (TP) sensor circuit provides a voltage signal that changes relative to the throttle blade angle. The signal voltage varies from less than 1 volt at closed throttle to about 4 volts at wide open throttle (WOT). The TP signal is one of the most important inputs used by the powertrain control module (PCM) for fuel control and most of the PCM-controlled outputs. The PCM monitors the throttle position and compares the actual throttle position from the TP sensor to a predicted TP value calculated from the engine speed. If the PCM detects an out of range condition, DTC P0121 is set.
Conditions for Running the DTC
| • | No TP or manifold absolute pressure (MAP) sensor DTCs set. |
| • | The engine is running. |
| • | The MAP is below 50 kPa in order to test for a fixed high TP sensor input. |
| • | The MAP is above 70 kPa in order to test for a fixed low TP sensor input. |
| • | The IAC position is between 0 and 130 counts. |
| • | The throttle angle is steady. |
Conditions for Setting the DTC
| • | The Predicted Throttle angle is not close to actual throttle angle. |
| • | The above conditions are present for longer than 10 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) during the first trip in which the diagnostic test has run and failed. |
| • | The PCM stores the conditions which were present when the DTC set as Freeze Frame and Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns the MIL off during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC clears after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC may be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed. |
Diagnostic Aids
Inspect for the following conditions:
| • | A damaged or sticking throttle plate or IAC valve A throttle plate. An IAC valve that allows too much airflow while the throttle should be closed may cause a high idle and DTC P0121 to be set. |
| • | A skewed MAP signal or malfunctioning MAP sensor. An incorrect MAP signal may cause the PCM to incorrectly calculate the predicted TP sensor value during high engine load situations. Check for an unusually low MAP reading. This condition may cause DTC P0121 to be set. |
| • | A faulty connection at PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections. |
| • | Inspect the wiring harness for damage. If the harness appears to be OK, observe the ECT display on the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the display indicates the location of the malfunction. |
If DTC P0121 cannot be duplicated, the information included in the Fail Records data may be useful in determining vehicle mileage since the DTC was last set. If the DTC occurs intermittently, performing the DTC P1122 and DTC P1121 Diagnostic Routines may isolate the cause of the malfunction.
Test Description
The number(s) below refer to the step number(s) on the Diagnostic Table:
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An incorrect manifold absolute pressure (MAP) signal may cause the powertrain control module (PCM) to incorrectly calculate the predicted throttle position (TP) sensor value during high engine load situations. Test for an unusually low MAP reading. This condition can cause DTC P0121 to be set.
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This vehicle is equipped with a PCM which utilizes an electrically erasable programmable read-only memory (EEPROM). When the PCM is replaced, the new PCM must be programmed
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||||
2 | Use a scan tool in order to observe the TP Angle reading while slowly opening the throttle. Does the TP Angle increase steadily and evenly through the specified range? | 0% - 100% | Go to Diagnostic Aids | |||||||
3 |
Is the TP sensor reading near the specified value? | 0 V | ||||||||
4 |
Does the voltage equal the specified value? | 5 V | ||||||||
5 | Test the following items:
If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems. Was a problem found? | -- | ||||||||
6 |
Was a problem found? | -- | ||||||||
7 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Did you complete the repair? | -- | -- | |||||||
|
Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming Replace the PCM. Did you complete the repair? | -- | -- | ||||||||
9 |
Does the scan tool indicate DTC P0121 Failed This Ign? | -- | System OK |
