Heater starts up fine, runs normally at boost, then reduces speed/heat a bit after about 10 mins, then as the cabin gets warmer it does not resduce heat any more. Was OK at start of season. No fault codes shown. Voltages OK. Wiring harness OK. Tried at various thermostat settings (including a very low temperature when it shouldn' have run at all, but it did.). Suggestions for doing checks? How do I check the themostat output (it is about 1.8Kohm)? Not on boat at present, have a simple multimeter for doing checks.