GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2004 🢒 XLR 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 DTC P1621

Circuit Description

While the vehicle is being driven, engine and transmission data accumulates in a random access memory (RAM) device, which cannot store data reliably when the transmission control module (TCM) powers down. Before the TCM powers down, the data is copied from RAM to an electrically-erasable programmable read-only memory (EEPROM) device, sometime referred to as non-volatile memory (NVM). Both the RAM and the EEPROM are inside the TCM. A normal function of the TCM programming is to test the EEPROM at TCM power-down by copying test data from RAM to the EEPROM and then comparing the two copies. If they do not match, DTC P1621 will set. DTC P1621 is a type A DTC.

Conditions for Running the DTC

The ignition is ON.

Conditions for Setting the DTC

The TCM detects a checksum error when the RAM and EEPROM test data are compared.

Action Taken When the DTC Sets
    • The TCM requests the ECM to illuminate the malfunction indicator lamp (MIL) when the Conditions for Setting the DTC are met.
    • The TCM commands maximum line pressure.
    • The TCM freezes transmission adaptive functions.
    • At the time of the first failure, the ECM records the operating conditions when the Conditions for Setting the DTC are met. The ECM stores this information as a Freeze Frame.
    • The TCM stores DTC P1621 in TCM history.

Conditions for Clearing the DTC
    • The ECM turns OFF the MIL after the sixth consecutive drive trip in which the TCM does not send a MIL illumination request.
    • A scan tool can clear the DTC.
    • The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without a non 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.

Diagnostic Aids

TCM reprogramming may cause P1621 to set.

Test Description

The number below refers to the step number on the diagnostic table.

  1. Performing this step helps verify that an external anomaly, such as Electromagnetic Interference (EMI), did not cause the code to set originally.

Step

Action

Value(s)

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

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

  3. Record the DTC Failure Records.
  4. Clear the DTC.
  5. Turn OFF the ignition for at least 30 seconds.
  6. Turn ON the ignition with the engine OFF.

Did DTC P1621 reset?

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Go to Step 3

Go to Intermittent Conditions in Engine Controls - 4.6L

3

Replace the TCM. Refer to Transmission Control Module Replacement .

Is the action complete?

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Go to Step 4

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4

Perform the following procedures in order to verify the repair:

  1. Select DTC.
  2. Select Clear Info.
  3. Turn ON the ignition with the engine OFF.
  4. Select Specific DTC.
  5. Enter DTC P1621.

Has the test run and passed?

 

Go to Step 5

Go to Step 2

5

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 - Vehicle in Vehicle DTC Information

System OK