HP EliteDesk 800 G1 Tower Service Manual page 215

Power LED flashes Red four times, once every second, followed by a two second pause, and the
computer beeps four times. (Beeps stop after fifth iteration but LEDs continue flashing.)
Cause
Power failure (power supply is overloaded).
The incorrect external power supply adapter is being used on
the All-in One or USDT.
Solution
1. If equipped with a voltage selector, check that the
voltage selector, located on the rear of the power supply
(some models), is set to the appropriate voltage. Proper
voltage setting depends on your region.
2. Open the access panel and ensure the power supply
cable is seated into the connector on the system board.
3.
Check if a device is causing the problem by removing
ALL attached devices (such as hard drives or optical
drives and expansion cards). Power on the system. If the
system enters POST, then power off and replace one
device at a time and repeat this procedure until failure
occurs. Replace the device that is causing the failure.
Continue adding devices one at a time to ensure all
devices are functioning properly.
4.
Replace the power supply.
5. Replace the system board.
The power supply adapter must be at the correct power rating
and use the Smart ID technology before the system will power
up. Replace the power supply adapter with the HP-supplied
power supply adapter.
Solving power problems
203