All combustion heater parts are now new. We even replaced the defective Overheat light (eyebrow panel). It's still happening. Combustion heater runs for 30 seconds, then overheat light comes on, heater cuts off. Some useful diagnostic there.
Last flight from upstate NY to Florida was on the coldest day of the year. Thankfully I had sun in my face the whole time, kept the cabin warm enough.
I think we have a direction to go.
Figure 13-1C, page H2 of Parts manual shows an "Inlet Valve", #4. This bypass allows air from the cabin air blower to divert around the heater. IF this valve is stuck open internally, or the actuator lever or cable is not correct, it just might be allowing air to bypass the heater even in part. This would cause the overtemp sensor to show the heat exchanger gradually increasing in temperature due to inadequate air flow.
You have to wonder what kind of engineers designed these systems. I can't imagine it being more obscure for a little airplane. It seems every part has multiple functions and is interconnected with other systems.
Still love my plane though, but I want it to keep me warm
Next update will be after we check the Inlet Valve.