Panasonic KX-TD500 System Reference Manual page 168

3.00 CPU RAM Testing
The KX-TD500 executes a testing of CPU RAM reading and writing when powered on.
If no error detected, the system continues booting to ON Line or OFF Line procedure.
When an error detected on CPU RAM, the LED1 of CPU card brinks. You need
repairing of CPU card in this case.
PUSH
LED1
Brinks when CPU RAM error is
detected.
RESET
MODE
SYTEM
CPU
2-F-8