Display shows F08; heater will not respond to thermostat changes.

Safety first: Turn off power before working on sensors or the control board.

• Compare sensor readings: With the heater off but pump running, the inlet and outlet temperatures should be very close. If the display shows a large difference, one sensor is likely faulty.
• Verify correct parts: Confirm that any recently replaced sensors are the correct Thermeau part numbers and not generic substitutes with different resistance curves.
• Inspect wiring routing: Ensure sensor wires are not bundled tightly with high-voltage or ignition wires, which can introduce electrical noise.
• Replace suspect sensor: Swap the suspected bad sensor with a known-good Thermeau sensor if available, or replace both inlet and outlet sensors to eliminate mismatch.
• Control board check: If new sensors do not resolve F08, the control board may be misreading inputs and should be evaluated or replaced by a qualified technician.