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
| • | No TP or MAP sensor DTCs set. |
| • | Idle air control is between 0 and 130 counts. |
| • | The engine is running. |
| • | MAP reading is less than 50 kPa (TP sensor high test) or greater than 60 kPa (TP sensor low test). |
| • | Throttle angle is steady. |
| • | Predicted throttle angle is not close to actual throttle angle. |
| • | The above conditions are present for at least 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 Information function or by disconnecting the PCM battery feed. |
Diagnostic Aids
The PCM will not complete the DTC P0121 diagnostic test until both the stuck high and the stuck low portions of the diagnostic have been completed. When verifying a repair or otherwise attempting to monitor the DTC P0121 diagnostic, refer to Conditions for Setting the DTC for the MAP signal levels required to complete both parts of the diagnostic.
Check for the following conditions:
| • | Vacuum hoses disconnected or vacuum leaks. |
| Visually and physically check the for the following conditions: |
| - | Disconnected vacuum hoses. |
| - | Damaged vacuum hoses. |
| - | Incorrect vacuum hoses routing. |
| - | Damaged MAP sensor seal. |
| - | Damaged fuel injector o-rings. |
| - | Damaged gasket at the throttle body, intake manifold, PCV valve cover, EGR pipes, or EGR valve mounting surface. |
| • | Incorrectly installed or malfunctioning PCV valve. |
| • | 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. |
| • | Incorrect 3X reference signal. |
| An incorrect 3X reference signal can cause the MAP signal to stop updating. This condition can cause the predicted TP sensor value to be calculated incorrectly and DTC P0121 to be set. A problem with the 3X reference signal should cause DTC P1374 to be set. If DTC P1374 is set with DTC P0121, diagnose DTC P1374 first. |
| • | Skewed MAP signal or malfunctioning MAP sensor. |
| An incorrect MAP signal may cause the PCM to incorrectly calculate the predicted TP sensor value. Check for an unusually low MAP reading during high engine load situations or a high MAP reading at idle. Either condition can cause DTC P0121 to be set. |
| • | Stuck open EGR valve. |
| An EGR valve that is stuck partially open may cause the TP sensor low test to fail and DTC P0121 to be set. A sticking EGR valve should cause DTC P1406 to be set. If DTC P1406 is set with DTC P0121, diagnose DTC P1406 first. |
| • | Poor connection at the PCM - Inspect harness connectors for the following conditions: |
| - | Backed out terminals. |
| - | Improper mating. |
| - | Broken locks. |
| - | Improperly formed or damaged terminals. |
| - | Poor terminal to wire connection. |
| • | Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the TP sensor 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:
-
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.
-
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.
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||
|
Important: If DTC P1635 is set, diagnose the other DTC first. Refer to DTC P1635 5 Volt Reference Circuit .
Is the MAP reading less than the specified value? | 65 kPa | |||||||||
3 |
Is MAP reading less than the specified value? | 65 kPa | ||||||||
4 |
Was the MAP signal circuit open or shorted? | -- | ||||||||
5 | Replace the MAP sensor. Refer 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?
| 0% 100% | Refer 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 wiring harness as necessary. Refer to Wiring Repairs in Electrical Diagnosis. Was a problem found? | -- | ||||||||
10 | Check the following items:
If a problem is found, repair wiring harness as necessary. Refer to Wiring Repairs 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 ign? | -- | System OK |
