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For 1990-2009 cars only
Makes 🢒 Saturn 🢒 2003 🢒 ION 🢒 Transmission/Transaxle 🢒 Automatic Transaxle - AF23-5 🢒 DTC P1791

Circuit Description

The engine control module (ECM) 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 ECM 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 ECM, then DTC P1791 will set. DTC P1791 is a type A DTC.

Conditions for Running the DTC
    • The ignition voltage is 8-18 volts.
    • The engine run time is 5 seconds or greater.
    • No other CAN errors are present.

Conditions for Setting the DTC

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

Action Taken When the DTC Sets
    • The TCM requests the ECM to illuminate the malfunction indicator lamp (MIL).
    • The TCM uses the ECM default throttle position value for determining shift speeds.
    • The TCM turns off the line pressure control solenoid valve (SLT). When the line pressure control solenoid valve is off, line pressure is at maximum and updates to the transmission adaptive pressure (TAP) memory cells are discontinued.
    • The ECM records the operating conditions when the Conditions for Setting the DTC are met. The ECM 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.

Conditions for Clearing the MIL/DTC
    • The ECM 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 - Engine Controls?

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Go to Step 2

Go to Diagnostic System Check - Engine Controls in Engine Controls - 2.2L (L61)

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 ECM Freeze Frame and the TCM Failure Records. Using the Clear Eng./Trans. DTC Info function erases the Freeze Frame and Failure Records from the ECM and the TCM.
       • Using the Clear Eng./Trans. DTC Info function erases stored DTCs in both the ECM and TCM.

  3. Record the DTC Failure Records.

Did you record any ECM Failure Records?

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Go to Diagnostic Trouble Code (DTC) List in Engine Controls - 2.2L (L61)

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?

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Go to Step 4

Go to Intermittent Conditions in Engine Controls - 2.2L (L61)

4

Did any other DTCs set?

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Go to Diagnostic Trouble Code (DTC) List in Engine Controls - 2.2L (L61)

Go to Step 5

5

Replace the ECM. Refer to Engine Control Module Replacement in Engine Controls 2.2L (L61).

Is the action complete?

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Go to Step 6

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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.

Has the test run and passed?

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Go to Step 7

Go to Step 2

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?

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Go to Diagnostic Trouble Code (DTC) List in Engine Controls - 2.2L (L61)

System OK