GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 GMC C Sierra - 2WD 🢒 DTC 🢒 DTC P2120

Circuit Description

The accelerator pedal position (APP) sensor 1 is a potentiometer type sensor with the following three circuits:

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

The control module provides the APP sensor a 5-volt reference circuit and a low reference circuit. The APP sensor then provides the control module a signal voltage proportional to pedal movement. The APP sensor 1 signal voltage is low at rest and increases as the pedal is depressed. When the control module detects that the APP sensor 1 signal or APP sensor 5-volt reference voltage is outside the predetermined range, this DTC sets.

Conditions for Running the DTC
    • DTCs P0601, P0602, P0606, P2108, or U0107 are not set.
    • The ignition switch is in the crank or run position.
    • The ignition voltage is more than 5.23 volts.

Conditions for Setting the DTC
    • The APP sensor 1 voltage is less than 0.24 volts or more than 4.49 volts.
        OR
    • The 5-volt reference is less than 4.54 volts or more than 5.21 volts.
    • One of the above conditions is present for more than 1 second.

Action Taken When the DTC Sets

    • The control module stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
    • If one or more APP sensor DTCs are set, the following occurs:
       - The control module commands Reduced Engine Power mode.
       - The APP indicated angle is limited to a predetermined value to limit the amount of throttle control.
       - OR
       - The APP indicated angle is limited to 0 percent. The control module only allows the engine to idle.
       - The message center displays Reduced Engine Power.

Conditions for Clearing the DTC
    • A current DTC Last Test Failed clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
    • Clear the DTC with a scan tool.

Diagnostic Aids
    • Inspect the throttle actuator control (TAC) module connectors for signs of water intrusion. If water intrusion occurs, multiple DTCs may set without any circuit or component conditions found during diagnostic testing.
    • When the TAC module detects a condition within the TAC system, more than 1 TAC system related DTC may set. This is due to the many redundant tests run continuously on this system. Locating and repairing 1 individual condition may correct more than 1 DTC. Disconnecting components during testing may set additional DTCs. Remember this if you review the stored information in Capture Info.
    • For an intermittent, refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This test isolates whether the short is to another TAC system circuit in the harness or within the TAC module.

    2.

  2. When the TAC module detects a condition within the TAC system, more than 1 TAC system related DTC may set. This is due to the many redundant tests run continuously on this system. Locating and repairing 1 individual condition may correct more than 1 DTC. Disconnecting components during testing may set additional DTCs. Remember this if you review the stored information in Capture Info.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

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

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important: If DTC P0120 or U0107 is also set, refer to the appropriate DTC for further diagnosis.

  1. Turn ON the ignition with the engine OFF, and with your foot OFF the accelerator pedal.
  2. Observe the accelerator pedal position (APP) sensor 1 voltage with a scan tool.

Does the scan tool indicate the APP sensor 1 voltage is within the specified values?

0.24-2.24 V

Go to Step 3

Go to Step 6

3

Depress the accelerator pedal to the wide open throttle (WOT) position.

Does the scan tool indicate APP sensor 1 voltage within the specified values?

0.24-4.49 V

Go to Step 4

Go to Step 6

4
  1. Turn OFF the ignition for 30 seconds.
  2. Turn ON the ignition, with the engine OFF.
  3. Select the diagnostic trouble code (DTC) option using the scan tool.
  4. Lightly touch and move the related engine wiring harnesses and connectors while monitoring the DTC information.

Did this DTC fail this ignition during the above test?

--

Go to Step 24

Go to Step 5

5
  1. Continue to observe the DTC Information.
  2. Depress the accelerator pedal to wide open throttle (WOT), then return the pedal to the rest position.

Did this DTC fail this ignition during the above test?

--

Go to Step 19

Go to Diagnostic Aids

6

Disconnect the APP sensor harness connector.

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

0 V

Go to Step 7

Go to Step 11

7

Connect a test lamp between the APP sensor 1 signal circuit and B+.

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

5 V

Go to Step 8

Go to Step 13

8

Test the APP sensor 1, 5-volt reference circuit for voltage with a DMM.

Does the DMM indicate voltage within the specified values?

4.54-5.21 V

Go to Step 10

Go to Step 9

9
  1. Turn OFF the ignition.
  2. Disconnect the throttle actuator motor harness connector.
  3. Remove the air inlet duct from the throttle body assembly.
  4. Turn ON the ignition, with the engine OFF.
  5. Rotate the throttle blade by hand to WOT and hold.
  6. Test the APP sensor 1, 5-volt reference circuit for voltage with a DMM.

Does the DMM indicate voltage within the specified values?

4.54-5.21 V

Go to Step 21

Go to Step 16

10
  1. Connect a fused jumper between the APP sensor 1 low-reference circuit and the APP sensor 1, 5-volt reference circuit.
  2. Observe the throttle position (TP) sensor 1 voltage parameter with a scan tool.

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

0 V

Go to Step 19

Go to Step 17

11
  1. Turn OFF the ignition.
  2. Disconnect the throttle actuator control (TAC) module harness connector containing the APP sensor circuits.
  3. Turn ON the ignition, with the engine OFF.
  4. Test the APP sensor 1 signal circuit for a short to voltage with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 12

12

  1. Turn OFF the ignition.
  2. Disconnect the other TAC module harness connector.
  3. Test for a short between the APP sensor 1 signal circuit and all other TAC module circuits with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 22

13
  1. Turn OFF the ignition.
  2. Disconnect the TAC module harness connector containing the APP sensor circuits.
  3. Test the APP sensor 1 signal circuit for an open or high resistance with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 14

14

Test the APP sensor 1 signal circuit for a short to ground with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 15

15
  1. Turn OFF the ignition.
  2. Disconnect the other TAC module harness connector.
  3. Test for a short between the APP sensor 1 signal circuit and all other TAC module circuits with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 22

16
  1. Turn OFF the ignition.
  2. Disconnect the TAC module connector containing the APP sensor circuits.
  3. Test the APP sensor 1, 5-volt reference circuit for the following conditions with a DMM:
    4. • An open
    • A short to ground
    • High resistance
    • Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 22

17
  1. Disconnect the TAC module connector containing the APP sensor circuits.
  2. Test the APP sensor 1 low-reference circuit for an open or high resistance with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 18

18

Test the TAC module ground circuit for an open or high resistance with a DMM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 22

19

Inspect for poor connections at the harness connector of the APP sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 20

20

Replace the APP sensor assembly. Refer to Accelerator Pedal Position Sensor Replacement .

Did you complete the repair?

--

Go to Step 25

--

21

Did DTC P0120 set while performing step 9?

--

Go to DTC P0120

Go to Step 22

22

Inspect for poor connections at the harness connector of the TAC module. Refer to Testing for Intermittent Conditions and Poor Connections and Repairing Connector Terminals in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 25

Go to Step 23

23

Replace the TAC module. Refer to Electronic Throttle Actuator Control Module Replacement .

Did you complete the repair?

--

Go to Step 25

--

24

Repair the intermittent condition as necessary. Refer to Connector Repairs and Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 25

--

25
  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 26

26

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK