PENTAIR MASTERTEMP 400 Service Heater Light On

Stack Flue Sensor (SFS)

The Stack Flue Sensor is a safety device that monitors the temperature of the exhaust gases to prevent heat exchanger damage. It frequently fails due to exposure to corrosive combustion byproducts, leading to inaccurate resistance readings that trigger the 'Service Heater' light. Replacing this sensor restores the control board's ability to verify safe operating temperatures during the combustion cycle.

SKU: 42002-0024S

Thermal Regulator

If the thermal regulator is stuck open or closed, it prevents the water from reaching the correct internal temperature or causes localized overheating within the heat exchanger. This leads to higher-than-normal stack temperatures, which then triggers the Stack Flue Sensor and the 'Service Heater' fault light.

SKU: 77707-0010

High Limit Switch Kit

These switches act as a secondary safety layer by monitoring water temperature directly on the manifold. If a switch becomes hypersensitive or 'weak' due to age, it will break the 24V circuit and illuminate the service light to prevent a boiling condition inside the header.

SKU: 42001-0063S