On a Jacuzzi spa, if powering down for 24 hours on a watchdog code does not solve the problem and it appears one of the sensors is bad, how can you determine good or bad? Can a continuity check be done?

You have to remove power completely from the unit, i.e. turn off the breaker for say 30 minutes. To check the sensors, use an OHM meter and each sensor, high limit and temp, should read about the same resistance.

Usually, a Watchdog problem can be solve only by changing the temp sensor. It can be the circuit board, but try to change the sensor before changing the board.