Sta-Rite Max-E-Therm and MasterTemp High Limit Switch, 42001-0063S

The Sta-Rite 42001-0063S is the high limit switch for Max-E-Therm SR-200, SR-333 and SR-400 pool heaters. It is a new OEM part. The same switch is also carried with the MasterTemp heater electrical system parts, so it turns up under both heater families.

What it does

The high limit switch is a safety cutout. It watches water temperature and opens the circuit if the heater pushes past the safe ceiling, stopping combustion before anything is damaged. It is doing its job every time the heater runs, which is why it eventually wears out.

Diagnosing it

A heater that fires and then shuts down once it is hot, or one that will not fire at all, often has a failed high limit switch. It sits in the electrical system alongside the thermistor, the transformer and the ignition control module, and it is the cheapest of that group to rule out first.