Circuit Description
The throttle position (TP) sensor circuit provides a voltage signal that changes relative to the throttle blade angle. The signal voltage will vary from less than 1 volt at closed throttle to more than 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.
Conditions for Running the DTC
| • | No TP or manifold absolute pressure (MAP) sensor DTCs are set. |
| • | The engine is running. |
| • | The MAP is less than 50 kPa in order to check for a fixed high TP sensor input. |
| • | The MAP is more than 70 kPa in order to check 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 the actual throttle angle. |
| • | The above conditions are present for more 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 runs and fails. |
| • | If equipped with traction control, the PCM will command the EBTCM via the serial data circuit to turn OFF traction control and illuminate the TRACTION OFF lamp. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has 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 a scan tool. |
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:
| • | Any disconnected vacuum hoses or vacuum leaks |
| Visually and physically check for the following conditions: |
| - | Any disconnected, damaged, or incorrectly routed vacuum hoses, refer to Emission Hose Routing Diagram . |
| - | A damaged MAP sensor seal, refer to Manifold Absolute Pressure Sensor Replacement . |
| - | Any damaged fuel injector O-rings, refer to Fuel Injector Replacement . |
| - | A damaged gasket at the throttle body, the intake manifold, the PCV valve cover, the EGR pipes, or the EGR valve mounting surface |
| • | An incorrectly installed or malfunctioning PCV valve--Refer to Crankcase Ventilation System Inspection . |
| • | A 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 (IAC) System Diagnosis . |
| • | An 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 Crankshaft Position (CKP) High to Low Resolution Frequency Correlation first. |
| • | A 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. Refer to Manifold Absolute Pressure Sensor Circuit Diagnosis . |
| • | A 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. Refer to DTC P1404 Exhaust Gas Recirculation (EGR) Closed Position Performance . |
| • | A faulty connection at the PCM--Inspect the harness connectors for backed out terminals, improperly formed or damaged terminals, and faulty terminal to wire connections. Use a corresponding mating terminal to test for proper terminal tension. |
| • | Inspect the wiring harness for damage. If the harness appears to be OK, observe the MAP 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 Failure 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 Throttle Position (TP) Sensor Circuit Intermittent Low Voltage and DTC P1121 Throttle Position (TP) Sensor Circuit Intermittent High Voltage diagnostic tables may isolate the cause of the malfunction.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On Board Diagnostic (OBD) System Check? | -- | ||
2 |
Important: If DTC P1635 5 Volt Reference Circuit is also set, perform that diagnostic first.
Does the TP Angle increase steadily and evenly from the first specified (closed throttle) value to the second specified (wide open throttle) value? | 0 percent 100 percent | Go to Diagnostic Aids | |
3 |
Does the TP voltage parameter approximately equal the specified value? | 0 V | ||
4 |
Is the TP voltage parameter near the specified value? | 5 V | ||
5 | Test the signal circuit of the TP sensor for a short to voltage. Refer to Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | ||
6 | Test the 5 volt reference circuit of the TP sensor for high resistance or a faulty connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems . Did you find and correct the condition? | |||
7 | Test the signal circuit of the TP sensor for high resistance or a faulty connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | |||
8 | Test the ground circuit of the TP sensor for high resistance or a faulty connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | ||
9 | Inspect for faulty connections at the harness connector of the TP sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | |||
10 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Did you complete the replacement? | -- | ||
11 |
Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is action complete? | -- | ||
Does the scan tool indicate DTC P0121 failed this ignition? | -- | System OK |
