GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The throttle position (TP) sensors 1 and 2 are located within the throttle body assembly. Each sensor has the following components:

    • A 5-volt reference circuit
    • A low reference circuit
    • A signal circuit

This provides the engine control module (ECM) with a signal voltage proportional to throttle plate movement. TP sensor 1 signal voltage at closed throttle is near the 5-volt reference and decreases as the throttle plate is opened. TP sensor 2 signal voltage at closed throttle is near the low reference and increases as the throttle plate is opened. If the ECM detects the TP sensor 1 signal voltage is not within the predicted range, DTC P0120 sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0120 Throttle Position (TP) Sensor 1 Circuit

Conditions for Running the DTC

    • The ignition switch is in the Crank or Run position.
    • DTC P0641 is not set.
    • The ignition voltage is greater than 5.23 volts.
    • DTC P0120 runs continuously when the above conditions are met.

Conditions for Setting the DTC

The ECM detects that the TP sensor 1 voltage is less than 0.27 volt or more than 4.67 volts for more than 0.4 second.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame and/or the Failure Records.
    • The control module commands the TAC system to operate in the Reduced Engine Power mode.
    • A message center or an indicator displays Reduced Engine Power.
    • Under certain conditions the control module commands the engine OFF.

Conditions for Clearing the MIL/DTC

    • The ECM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic runs and passes.
    • 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.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Are DTCs P0122 and P0123 also set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3

  1. Turn ON the ignition, with the engine OFF.
  2. Observe the throttle position (TP) sensor voltage with the accelerator pedal in the rest position with a scan tool.

Does the scan tool indicate voltage less than the first value or greater than the second value?

0.27 V

4.67 V

Go to Step 6

Go to Step 4

4

Is DTC P2135 also set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 5

5

  1. Observe the Freeze Frame/Failure Records for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 6

Go to Testing for Intermittent Conditions and Poor Connections

6

  1. Turn OFF the ignition.
  2. Disconnect the throttle body harness connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the TP Sensor 1 Voltage parameter with a scan tool.

Does the scan tool indicate voltage at the specified value?

0 V

Go to Step 7

Go to Step 12

7

  1. Connect a fused jumper wire between the TP sensor 5-volt reference circuit and the TP sensor 1 signal circuit at the throttle body harness connector.
  2. Observe the TP Sensor 1 Voltage parameter with a scan tool.

Does the scan tool indicate the TP sensor 1 voltage at the specified value?

5 V

Go to Step 8

Go to Step 9

8

Probe the TP sensor low reference circuit with a test lamp connected to B+.

Does the test lamp illuminate?

--

Go to Step 18

Go to Step 14

9

Measure the voltage of the TP sensor 1 5-volt reference circuit with a DMM.

Does the DMM indicate voltage at the specified value?

5 V

Go to Step 11

Go to Step 10

10

Does the DMM indicate voltage less than the specified value on the TP sensor 1 5-volt reference circuit?

5 V

Go to Step 15

Go to Step 17

11

Test the TP sensor 1 signal circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 13

12

Test the TP sensor 1 signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 20

13

Test the TP sensor 1 signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 20

14

Test the TP sensor 1 low reference circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 20

15

Test the TP sensor 1 5-volt reference circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 16

16

Test the TP sensor 1 5-volt reference circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 20

17

Test the TP sensor 1 5-volt reference circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs.

Did you find and correct the condition?

--

Go to Step 22

--

18

Test for an intermittent and for a poor connection at the throttle body. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 19

19

Replace the throttle body assembly. Refer to Throttle Body Assembly Replacement .

Did you complete the replacement?

--

Go to Step 22

--

20

Test for an intermittent and for a poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 21

21

Replace the ECM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

--

Go to Step 22

--

22

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 23

23

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

System OK