GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Equinox 🢒 Brakes 🢒 Park Brake 🢒 DTC P1791

Circuit Description

The powertrain control module (PCM) sends throttle position data to the transmission control module (TCM). The TCM uses this data to modify shift speeds. The data is sent to the TCM through a communication network called the controller area network (CAN). Two circuits are used to communicate CAN data between the PCM and TCM. A fault in the CAN will not cause DTC P1791 to set by itself. If a CAN fault occurs, other DTCs will set before DTC P1791.

When the TCM receives invalid throttle position data from the PCM, then DTC P1791 will set. DTC P1791 is a type A DTC.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1791 Throttle/Pedal Position Signal

Conditions for Running the DTC
    • The ignition voltage is 10.2-15.5 volts.
    • The engine speed is 400 RPM or greater.
    • No other CAN errors are present.
    • Trans. protection mode is OFF.
    • The ignition is ON for 3.0 seconds or greater.

Conditions for Setting the DTC

The PCM sends no valid throttle position data to the TCM for 4 seconds.

Action Taken When the DTC Sets
    • The TCM requests the PCM to illuminate the malfunction indicator lamp (MIL).
    • The TCM uses the PCM default throttle position value for determining shift speeds.
    • The PCM records the operating conditions when the Conditions for Setting the DTC are met. The PCM stores this information as Freeze Frame and Failure Records.
    • The TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as Failure Records.
    • The TCM stores DTC P1791 in TCM history.
    • The TCM turns the trans. protection mode ON.

Conditions for Clearing the MIL/DTC
    • The PCM turns off the MIL after the third consecutive drive trip in which the TCM does not send a MIL illumination request.
    • A scan tool can clear the MIL/DTC.
    • The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without an emission-related diagnostic fault occurring.
    • The TCM cancels the DTC default actions when the ignition is OFF long enough in order to power down the TCM.

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle in Vehicle DTC Information?

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Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2
  1. Install a scan tool.
  2. Turn ON the ignition with the engine OFF.

    3. Important: 

       • Before clearing the DTC, use the scan tool in order to record the PCM Freeze Frame and the TCM Failure Records. Using the Clear Eng./Trans. DTC Info function erases the Freeze Frame and Failure Records from the PCM and the TCM.
       • Using the Clear Eng./Trans. DTC Info function erases stored DTCs in both the PCM and TCM.

  3. Record the DTC Failure Records.

Did you record any PCM Failure Records?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

Go to Step 3

3
  1. Clear the DTC.
  2. Turn the ignition OFF for at least 30 seconds.
  3. Start, and allow the engine to idle.

Did DTC P1791 reset?

--

Go to Step 4

Go to Intermittent Conditions in Engine Controls - 3.4L

4

Did any other DTCs set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

Go to Step 5

5

Replace the PCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Is the action complete?

--

Go to Step 6

--

6

Perform the following operation to verify the repair:

  1. Select DTC on the scan tool.
  2. Select Clear Info.
  3. Start the engine and vary the throttle.
  4. Select Specific DTC.
  5. Enter DTC P1791.

Does the DTC reset?

--

Go to Step 2

Go to Step 7

7

With the scan tool, observe the stored information, capture info and DTC info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

System OK