Circuit Description
WARNING: This page is about a different car, the 2005 Cadillac STS. However, it is still accessible from the selected car via links, so may be relevant.
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.