HP RP7 Model 7800 Maintenance And Service Manual page 163

Power LED flashes red five times, once every second, followed by a two second pause, and the computer beeps
five times. (Beeps stop after fifth iteration but LEDs continue flashing.)
Cause
Memory is installed incorrectly or is bad.
Solution
CAUTION: To avoid damage to the DIMMs or the system
board, you must unplug the computer power cord before
attempting to reseat, install, or remove a DIMM module.
1. Reseat DIMMs. Power on the system.
2. Replace DIMMs one at a time to isolate the faulty
module.
NOTE: DIMM1 must always be installed. DIMM1 must
be installed before DIMM3.
3. Replace third-party memory with HP memory.
4. Replace the system board.
Solving hardware installation problems 153