To repair a spa tub, first diagnose the problem. Common problems include spa tubs that just do not run, continuously move on low speed or leak. Once you have identified the problem, refer to the spa tub's user manual for self-repairs. If the manual's troubleshooting tips do not work, call a professional plumber.
If the spa tub does not run, has no power and won't work at all, check the circuit breaker for incoming power to the tub. If the problem persists, test and reset the GFCI, check the fuse, and check the voltage at the high limit contactor. If the problem remains, you may need to replace or repair the circuit board or internal system.
If the spa tub is continuously running on low speed, make sure that the timer is off and the thermostat is fully counter-clockwise. Select high speed, and listen for the motor switching over. If it doesn't switch over, try rotating the timer. If it does, return to low speed, and disconnect one of the wires from the contactor coil. If this fixes the problem, replace the wire; if not, replace the contactor.
If the spa tub leaks, check the union fittings at the spa pump and heater. Inspect the heater, valves and pipes to locate the leak, and use a sealant to repair the damage. If you cannot find the leak, check all connections throughout the spa tub, making sure all pipes are properly fitted together.