Cannot find the fault code . There are vidoes on u tube saying how to reset thr code By pressing the reset button . If this does not work youcan ccontact vaillent on there web page . I have a mate who works for them i will ask him as well
The P8 fault can mean many things when it pops up on your Mitsubishi Electric wall controller. The P8 fault is activated when the indoor coil sensor does not change the temperature for a set amount of time after the compressor is (or should be) switched on.
Beware when working with electrical equipment. It may cause electric shock.
Check the power supply wires, connections with heating coils. If they are ok then possibly heating coils require replacement.
Domestic Central Heating, Dodge Caravan, well pump or a Pool Heater, or something else.
All questions need 'make model (what it is)' as first 3 words.
With the exception of the Dodge Caravan, pumps have a small bleed screw.
Google
make model bleed pump
or if it is an above ground well pump
make model prime pump
This is heavily weighted towards vehicles, but you will get the ideal of the power of Google!
https://www.google.com/search?q=%E2%80%8Bmake+model+bleed+water+pump
..
Call for Service. The timer is fine, the boiler has failed. I presume it is gas fired, which means that only gas registered service techs can work on it.