GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 1996 🢒 Century 🢒 DTC 🢒 DTC P0121 Throttle Position (TP) Sensor Performance

Object Number: 28306 Size: LF
Engine Controls Components
ECT, MAP, TP, IAT, and A/C Systems
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 active TP sensor or MAP sensor 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:

    • 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 the DTC P1122 and DTC P1121 Diagnostic Charts may isolate the cause of the fault.

Test Description

Number(s) below refer to the Step number(s) on the Diagnostic Chart:

  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.

  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 System Check performed?

--

Go to Step 2

Go to Powertrain OBD System Check

2

Ignition ON, observe MAP reading on scan tool Eng. 1 data list.

Is MAP reading less than the specified value?

55 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 Greater than the specified value?

95 kPa

Go to Step 5

Go to Step 4

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 section 8A-5).

Was the MAP signal circuit open or shorted?

--

Go to Step 13

Go to Step 12

5

Replace the MAP sensor.

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%

Refer 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:

  1. TP signal circuit for a short to voltage.
  2. TP sensor ground circuit for high resistance between the PCM and the TP sensor.
  3. TP sensor ground circuit for a poor connection.
  4. If a problem is found, repair wiring harness as necessary (refer to Repair Procedures in Electrical Diagnosis section 8A-5).

Was a problem found?

--

Go to Step 13

Go to Step 12

10

Check the following items:

  1. TP signal circuit or 5 volt reference A circuit for a poor connection.
  2. TP signal circuit or 5 volt reference A circuit for high resistance between the PCM and the TP sensor.
  3. If a problem is found, repair wiring harness as necessary (refer to Repair Procedures in Electrical Diagnosis section 8A-5).

Was a problem found?

--

Go to Step 13

Go to Step 12

11

Replace the TP sensor.

Is action complete?

--

Go to Step 13

--

12

Replace the PCM.

Important: Replacement PCM must be programmed. Refer to On-Vehicle Service in Powertrain Control Module and Sensors for procedures.

Is action complete?

--

Go to Step 13

--

13
  1. Review and record scan tool Fail Records data.
  2. Clear DTCs.
  3. Operate vehicle within Fail Records conditions as noted.
  4. Using a scan tool, monitor Specific DTC info for DTC P0121.

Does scan tool indicate DTC P0121 failed this ignition?

--

Go to Step 2

Repair complete