Heater shows F18 and will not run; may be associated with freeze protection or low ambient temperatures.

Safety first: Disconnect power before working on sensors or wiring.

• Locate ambient sensor: On some Thermeau models, a separate sensor monitors outdoor or cabinet temperature for freeze protection. Identify its location from the service manual.
• Inspect sensor and cable: Check for physical damage, rodent chewing, or disconnection.
• Check reading: Compare the displayed ambient temperature (if available) to actual outdoor temperature. A large discrepancy indicates a bad sensor.
• Replace sensor: Install a new Thermeau ambient/freeze sensor if readings are incorrect or F18 persists.
• Board check: If a known-good sensor does not clear F18, the control board input may be defective and require replacement.