Spas use TWO sensors.
The Temperature and the Overheat or High Limit sensor.
change resistance values as the water heats or
cools. The change in resistance tells the board the
temperature of the water and the heater. If the
sensor itself gives faulty or no resistance values,
the spa will not operate or operates wrong. The
board is smart enough that it will typically tell
you by sensor error messages if the values are out
of range by giving you a sensor error code on the
The Temperature sensor
must either be IN the water or IN a housing that is
in the water. Most common design is in the
water, usually where the filter resides. This sensor
tells the board actual temp. If the topside reading
on the board is off, then its usually this sensor
(could be the board). If the board sees the water
get too hot, it will attempt to cool the spa or shut
The OH or High Limit
sensor has one purpose in life. It sits next to the
heater and watches the heater closely. If the heater
starts to overheat, the board sees the temp on the
OH sensor and will shut the spa down. So if you get
an OH error, either the heater IS overheating OR the
sensor has failed.
A fault error
code of --- is likely the temp sensor.