Transmission Diagnosis

DTC P1621 (3.0L)

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, sometimes 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 sets. 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 check-sum 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).
^ The TCM turns off the pressure control solenoid. When the pressure control solenoid 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 P1621 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.

Diagnostic Aids
TCM reprogramming may cause P1621 to set.

Test Description
The number below refers to the step number on the diagnostic table.
2. Performing this step helps verify that an external anomaly, such as Electromagnetic Interference (EMI), did not cause the code to set originally.

Steps 1 - 5:

DTC P1621 (3.0L)