The heater displays F13 and shuts down; this may coincide with very cold ambient conditions.

Safety first: Disconnect power before servicing sensors.

• Locate freeze sensor: The freeze or ambient sensor is typically clipped to the heater frame or mounted near the air intake.
• Check mounting: Ensure the sensor is not touching hot surfaces or exposed to direct sunlight, which can cause false readings.
• Inspect wiring: Look for damage or loose connections. Reseat the connector at the control board.
• Replace sensor: If F13 persists and the displayed ambient temperature is clearly wrong, replace the freeze sensor with an Aquacomfort-compatible part.