912ULS Coolant Temp Sensor Error

Those sensors are down in the coolant and don't measure the metal head temp and will be so many degrees different. During this time of year (summer) and depending on your engine cowl setup you m,ay reach 248F. It is perfectly normal this time of year to see 230F - 245F in climb and depending on your take off technique. This is in the yellow arc and only means keep an eye on me and don't let me go into the red. Yellow is just an attention getter so you'll pay attension and also have an idea what's green normal, yellow normal high before red. That said when you level out the temps should drop back down around 220F +/- for this time of year. Colder months will get you cooler temps.

Some people do get to 250F this time of year, but this can be your engine cowl setup and or how steep or hard you climb. Taking off flatter will imporve your cowl airflow with more speed and straighter access into the cowl.

Thanks Roger. Yes, it's a suffix-01 912 so measures coolant and not CHT. I should have mentioned that this condition is on the ground at extended idle. It's a Searey and is uncowled. Seareys typically don't see more than 160-190F at idle ever, even in 100F weather. I'm in the Pacific NW where we have mild Summers, 75-80 OAT.  My infrared thermometer is measuring cylinder head temps at the sensor location that are around 200F when the coolant temp readings on the EMS are 240-250F. 

I can only suggest to check water circulation, pump and both caps to make sure they are holding pressure. Check radiator internal blockage.

 But, first try a flush with dex cool. Get the concentrated one and mix it yourself with distilled water 50 to 50. Check condition of old fluid and amount. As you know do this flush when cold and repeat flush with distilled water a couple of gallons before putting dex cool 50/50 in. Check flow through radiator during repeated flush with distilled water to make sure there is no blockage. Do not use regular water. Distilled water is not expensive and most grocery stores carry them. Buy, quality Dex cool concentrated antifreeze. 

My engine is also uncowled and my reading is low all the time. 

Have you tried immediately draining some fluid after shut down in a metal cup from radiator drain plug when temp high and measure the temp (do not open caps).  Be careful since it will Be very hot fluid. Take steps not to burn yourself. Wear proper protective gear and face shield and have bucket of ice water near by just in case. I know it will be difficult on Searey since you will be working on a ladder. 

First check the fluid level at the spider by opening cap and making sure there is fluid. If cap is bad it will not take fluid or give fluid to the reservoir. 

IF fluid ok drain it and fill with distilled water only. Run engine and see how temp is. The antifreeze only keeps fluid from freezing. Water is more important than antifreeze to keep heads cool. It is possible water somehow evaporated from antifreeze and you are getting hot spots near sensors. 

Thanks. I should have mentioned this is a brand new build. Yes, I drained the entire cooling system and vacuum filled it with Dex Cool 50/50. CHT's are in the normal range per direct meaasurement with infrared thermometer, so coolant temps should be also. However, coolant temps are reading much higher than CHT's on EMS. It should be the other way around. Also, I measured the coolant sensor resistances and the Dynon EMS is interpreting them correctly when converting them to temperature,  Everything points to the stock sensors.