Circuit Description
A normal function of the transmission control module (TCM) programming is to perform an internal check that verifies the EEPROM data. When the TCM cannot read the EEPROM data, then DTC P1621 will set. DTC P1621 is a type B DTC.
Condition for Running the DTC
The ignition switch is in the RUN position.
Condition for Setting the DTC
TCM is unable to read to EEPROM data.
Action Taken When the DTC Sets
| • | Early production - The TCM illuminates the service transmission lamp (STL) and sends a MIL request to the ECM on the second consecutive drive trip that the diagnostic runs and fails. The ECM then illuminates the MIL. |
| • | Late production - The TCM flashes the Sport Mode Lamp and sends a MIL request on the second consecutive drive trip that the diagnostic runs and fails. The TCM then illuminates the MIL. |
| • | The ECM records the operating conditions in the Freeze Frame at the time of the MIL request from the TCM. |
| • | The transmission operates in the default mode (maximum line pressure, command 4th gear, inhibit TCC, freeze shift adapts). |
Conditions for Clearing the MIL/DTC
| • | Early production - The TCM turns off the STL as soon as the fault is no longer present. |
| • | Late production - The TCM stops flashing the sport mode lamp as soon as the fault is no longer present. |
| • | The ECM turns off the MIL after three consecutive drive trips during which the TCM sends no MIL request. |
| • | A History DTC clears after forty consecutive warm-up cycles, if no failures are present by this diagnostic or any other emission related diagnostic. |
| • | The scan tool clears the MIL/DTC. |
| • | The TCM cancels the DTC default mode actions when the fault no longer exists and the ignition switch is Off long enough in order to power down the TCM. |
Diagnostic Aids
Vehicles equipped with Fuel Cap Lamps utilize the Sport Mode lamp to indicate a transmission malfunction. In these vehicles, the Sport Mode Lamp flashes at 200 millisecond intervals when the TCM commands the lamp ON.
Vehicles not equipped with Fuel Cap Lamps are equipped with a dedicated Service Transmission Lamp (STL) on the Instrument Panel Cluster (IPC). The STL illuminates steadily when the TCM commands the lamp ON.
If TCM reprogramming has recently been completed, this could be the reason for P1621 to set.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
Important: Before clearing the DTC, use the Scan Tool in order to record the Failure Records. Using the Clear Info function erases the Failure Records from the ECM/TCM. Did DTC P1621 reset? | -- | Go to Diagnostic Aids | ||
3 | Replace the TCM. Refer to Transmission Control Module Replacement . Is the replacement complete? | -- | -- | |
4 | Perform the following procedures in order to verify the repair:
Did DTC P1621 reset? | System OK |
