Description

Sta-Rite 42001-0053S is the replacement thermistor for the Max-E-Therm SR-200, SR-333 and SR-400 heaters. The same sensor is part of the MasterTemp heater electrical system.

What it does

The thermistor is the heater control board eyes on water temperature. Its resistance changes with temperature, the board reads that, and everything downstream follows: what the display shows, when the burner fires, when the heater satisfies the setpoint and shuts down.

When it is the problem

A drifting or failed thermistor makes the heater behave in ways that look like other faults. Temperatures that read nothing like the pool, a heater that will not fire even though the pool is cold, or short cycling near setpoint all point at the sensor. It is an inexpensive part to change and a common one to find at fault.

Fitting

It is a direct OEM replacement, so it goes back into the same location on the same wiring. Kill power at the breaker before opening the heater.