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 Setting the DTC
| • | The engine is running. |
| • | MAP reading is below 55 kPa. |
| • | Predicted throttle angle is not close to actual throttle angle. |
| • | Above conditions are present for longer than 10 seconds. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (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 Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the MIL 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. |
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 airfow while the throttle should be closed may cause a high idle and DTC P0121 to be set. Refer to Idle Air Control (IAC) System Check . |
| • | Skewed MAP signal or faulty 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 fault. |
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 DTC P1122 or DTC P1121 Diagnosis may isolate the cause of the fault.
Test Description
Number(s) below refer to the Step number(s) on the Diagnostic Table:
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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.
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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 being replaced, the new PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming .
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||
|
Important: If DTC P1635 is also set, diagnose DTC P1635 first. Refer to DTC P1635 5 Volt Reference Circuit .
Is the MAP reading less than the specified value? | 65 kPa | |||||||||
3 |
Is the MAP reading less than the specified value? | 65 kPa | ||||||||
4 |
Was a problem found? | -- | ||||||||
5 | Replace the MAP sensor. Go to Manifold Absolute Pressure Sensor Replacement . Is action complete? | -- | -- | |||||||
6 | 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?
|
| Go to Diagnostic Aids | |||||||
7 |
Is the TP sensor reading near the specified value? | 0V | ||||||||
8 |
Is the TP sensor reading at the specified value? | 5V | ||||||||
9 | Check the following items:
If a problem is found, repair the wiring harness as necessary. Refer to Repair Procedures in Electrical Diagnosis. Was a problem found? | -- | ||||||||
10 | Check the following items:
If a problem is found, repair wiring harness as necessary. Refer to Repair Procedures in Electrical Diagnosis. Was a problem found? | -- | ||||||||
11 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Is action complete? | -- | -- | |||||||
Replace the PCM. Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming | -- | -- | ||||||||
13 |
Does the scan tool indicate DTC P0121 failed this ignition? | -- | System OK |
