Hayward HeatPro HP50TA Troubleshooting Manual page 29

See notes 1 and 2 below
Replace HP switch.
Note 1
Heater runs for a while then shuts off and shows 'HI or HP' on
display. Low water flow is normally the problem. Check filter
and pump. A common problem when running unit on spa
exists when spa temperature of about 100⁰ F is reached and
the unit shuts off with the 'HI or HP' fault. At higher outdoor
and water temperatures a higher flow rate may be required
for proper operation. The unit requires a minimum of 30
GPM, but may require more under these conditions.
Flow Chart
Does heat pump run for a
YES
short time, then cut off
showing Hi or HP?
Check Ohm's reading on HP switch.
NO
Reading 0 Ohms?
Replace control board
HI/HP
HI(HP)
YES
Check all wires and
connections
from HP switch to
control board. Wires
and connections OK?
Correct wiring
HI/HP after the heat pump has run for a few seconds could
be an indication of a failed TXV. If the failure is caused
by the TXV, the low side (suction) pressure will fall rapidly once
the compressor is running, and the high side (head) pressure
YES
NO
Note 2
will rise rapidly above 590 PSI.