Circuit Description
The Throttle Position (TP) sensor circuit provides a voltage signal that changes relative to throttle blade angle. The signal voltage will vary 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 PCM for fuel control and most of the PCM-controlled outputs. The PCM monitors throttle position and compares actual throttle position from the TP sensor to a predicted TP value calculated from engine speed. If the PCM detects an out of range condition, DTC P0121 will be set.
Conditions for Running the DTC
| • | No TP or MAP sensor DTCs set. |
| • | Engine is running. |
| • | MAP is below 50 kPa to check for a fixed high TP sensor input. |
| • | MAP is above 70 kPa to check for a fixed low TP sensor input |
| • | IAC Position is between 0 and 130 counts. |
| • | Throttle angle is steady. |
Conditions for Setting the DTC
| • | 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 will illuminate the MIL during the first trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed. |
Diagnostic Aids
Check for the following conditions:
| • | Damaged or sticking throttle plate or IAC valve A throttle plate or IAC valve that allows too much airflow while the throttle should be closed may cause a high idle and DTC P0121 to be set. Refer to Idle Air Control System Check for diagnosis. |
| • | 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 can cause DTC P0121 to be set. |
| • | Poor connection at PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. |
| • | Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the ECT display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the malfunction. |
If DTC P0121 cannot be duplicated, the information included in the Fail Records data can be useful in determining vehicle mileage since the DTC was last set. If it is determined that the DTC occurs intermittently, performing the DTC P1122 and DTC P1121 Diagnostic Charts may isolate the cause of the malfunction.
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table:
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||||
2 | Observe the TP Angle reading on the scan tool while slowly opening the throttle. Does TP Angle increase steadily and evenly from the closed throttle value to the wide open throttle value?
| 0% 100% | Go to Diagnostic Aids | |||||||||
3 |
Is the TP sensor reading near the specified value? | 0V | ||||||||||
4 |
Is the TP sensor reading at the specified value? | 5V | ||||||||||
5 |
Was a problem found? | -- | ||||||||||
6 |
Was a problem found? | -- | ||||||||||
7 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Is action complete? | -- | -- | |||||||||
|
Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming Replace the PCM. Is action complete? | -- | -- | ||||||||||
9 |
Does the scan tool indicate DTC P0121 failed this ignition? | -- | System OK |
