For 1990-2009 cars
Engine Controls Components
A/C Presure, Fuel Tank Pressure, TP, MAP, IAT, ECT Sensors
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

### 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 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

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. Refer to Emission Hose Routing Diagram .

- | Damaged vacuum hoses.

- | Incorrect vacuum hoses routing. Refer to Emission Hose Routing Diagram .

- | Damaged MAP sensor seal. Refer to Manifold Absolute Pressure Sensor Replacement .

- | Damaged fuel injector O-rings. Refer to Fuel Injector Replacement .

- | Damaged gasket at the throttle body, intake manifold, PCV valve cover, EGR pipes, or EGR valve mounting surface.

• | Incorrectly installed or malfunctioning PCV valve. Refer to Positive Crankcase Ventilation 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 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 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 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:

1. 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.

2. 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 PCM Replacement and Programming Procedures in Powertrain Control Module (PCM) and Sensors.

#### DTC P0121 - TP System Performance

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Important: If any of the following DTCs are set, diagnose the other DTC first. DTC P1374 Crankshaft Position (CKP) High to Low Resolution Frequency Correlation . DTC P1406 Exhaust Gas Recirculation (EGR) Position Sensor Performance . DTC P1635 5 Volt Reference Circuit .

1. Turn ON the ignition switch, engine not running.
2. Observe the MAP reading on the scan tool.

Is the MAP reading less than the specified value?

65 kPa

Go to Step 3

Go to Step 6

3 |

1. Disconnect the MAP sensor.
2. Connect a test light between the 5 volt reference A circuit and the MAP signal circuit at the MAP sensor harness connector
3. Observe MAP reading on the scan tool.

Is MAP reading less than the specified value?

65 kPa

Go to Step 4

Go to Step 5

4 |

1. Check the MAP signal circuit between the PCM and the MAP sensor for an open, short to ground, or short to the MAP ground circuit.
2. If the MAP signal circuit is open or shorted, repair it as necessary. Refer to Repair Procedures in Electrical Diagnosis.

Was a problem found?

--

Go to Step 13

Go to Step 12

5 | Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement . Refer to Manifold Absolute Pressure Sensor Replacement . Is action complete? | -- | Go to Step 13 | --
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?

• | Closed throttle value is the first Value.

• | Wide open throttle value is the second Value.

0%

100%

Go to Diagnostic Aids

Go to Step 7

7 |

1. Disconnect the TP sensor.
2. Observe the TP sensor reading on the scan tool.

Is the TP sensor reading near the specified value?

0V

Go to Step 8

Go to Step 9

8 |

1. Connect a test light between the 5 volt reference A circuit and the TP sensor signal circuit at the TP sensor harness connector.
2. Observe the TP sensor reading on the scan tool.

Is the TP sensor reading at the specified value?

5V

Go to Step 11

Go to Step 10

9 | Check the following items:

• | TP signal circuit for a short to voltage

• | TP sensor ground circuit for high resistance between the PCM and the TP sensor

• | TP sensor ground circuit for a poor connection

If a problem is found, repair wiring harness as necessary. Refer to Electrical Diagnosis.

Was a problem found?

--

Go to Step 13

Go to Step 12

10 | Check the following items:

• | TP signal circuit or 5 volt reference A circuit for a poor connection.

• | TP signal circuit or 5 volt reference A circuit for high resistance between the PCM and the TP sensor.

If a problem is found, repair wiring harness as necessary. Refer to Electrical Diagnosis.

Was a problem found?

--

Go to Step 13

Go to Step 12

11 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Is action complete? | -- | Go to Step 13 | --
12 | Replace the PCM. Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming | -- | Go to Step 13 | --
13 |

1. Review and record scan tool Fail Records data.
2. Clear DTCs
3. Operate the vehicle within Fail Records conditions.
4. Using a scan tool, monitor Specific DTC info for DTC P0121.

Does the scan tool indicate DTC P0121 failed this ign?

--

Go to Step 2

System OK