The PCM performs the checksum test to monitor the integrity of the non-volatile information across ignition cycles. The PCM uses an Electrically Erasable Programmable Read Only Memory (EEPROM) to store this information at key OFF. Before storing the information, the PCM calculates a checksum then stores the value along with the information. At key ON, the PCM retrieves the information from the EEPROM and places it in RAM. After retrieval, the PCM conducts a checksum of the information in RAM and compares this value to the value stored in the EEPROM at the previous key OFF. If the checksums do not match, DTC P0603 will set. Also, during operation, the PCM maintains a checksum on a certain section of the RAM that contains critical information. If at any time this running checksum fails, DTC P0603 will set.

The PCM detects a calculated checksum that does not match previous checksum.

If PCM power or ground is lost during vehicle operation or during key down, DTC P0603 will set. The PCM keeps a continuous check on the non-volatile memory. If the memory changes, a PCM memory reset occurs. An intermittent loss of power or ground to the PCM will cause a reset.