Check to ensure the outdoor heat pump has power if you see a P8 error code. Both the indoor and outdoor heat pumps are 208-230/1/60 volt units, and the P8 error code results if the outdoor unit has no power, even if the indoor unit has power.
Verify that you do not have an open pipe temperature thermistor and that it fits securely into the refrigerant pipe well. An open pipe temperature thermistor means there may be an open circuit from a blown fuse or a manually disconnected thermistor.
Although the outdoor board tries to maintain a 95-degree Fahrenheit temperature, your heat pump will turn itself off if the discharge temperature gets too low or too high. For instance, if the pipe temperature drops to 9 degrees F below room temperature, the error code P8 will result.
Check the two volt DC (VDC) wires connected to the indoor and outdoor heat pumps by ensuring they fit securely rather than loosely in their terminals and that the wires do not cross. The number-one wire of the indoor unit must lead to the number-one wire of the outdoor unit.
The number-two wire of the indoor and outdoor units must also match, otherwise your Mitsubishi's heat pump stops operating and the P8 error code appears nine minutes later.