from the icd2 q&a

Q: MPLAB ICD 2 responds with "Target not in debug mode error." What does
this mean?

A: Usually this means that MPLAB ICD 2 cannot communicate with the debug
executive. The debug executive can only be downloaded by programming the
target PICmicro MCU with the user's application from the Debugger>Program
menu selection. There are other reasons that the debug executive might not
be able to communicate, such as target clock or power supply problems. Check
the configuration bits to see that the "Background Debug" is enabled. Look
at Configure>Configuration Bits to make sure that the Watchdog TImer is
disabled, code protection is turned off, and that the oscillator setting is
correct.

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