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.
This diagnostic procedure supports the following DTC:
DTC P1621 Transmission Control Module (TCM) Long Term Memory Performance
The ignition is ON.
The TCM detects a checksum error when the RAM and EEPROM test data are compared.
• | The TCM requests the ECM to illuminate the malfunction indicator lamp (MIL) when the Conditions for Setting the DTC are met. |
• | The TCM commands a soft landing to 2nd gear. |
• | The TCM commands maximum line pressure. |
• | The TCM forces the TCC OFF. |
• | The TCM inhibits TCC engagement. |
• | 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. |
• | 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 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. |
TCM reprogramming may cause DTC P1621 to set.
The number below refers to the step number on the diagnostic table.
Step | Action | Value(s) | Yes | No | ||||
---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||
Important:
Did DTC P1621 reset? | -- | Go to Step 3 | Go to Intermittent Conditions in Engine Controls - 2.2L | |||||
3 | Replace the TCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Is the action complete? | -- | Go to Step 4 | -- | ||||
4 | Perform the following procedures in order to verify the repair:
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? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | System OK |