Does anyone know what the btu is for a aquacal pool heater model #120AHDEBNC or serial #1071402-E06

To find items like this Ken you need the proper alpha-numeric model numbers rather than descriptions like '100,00 BTU' or 'Titanium'. The search queries are simply

make model manual

and

make model parts

Less words is best.

https://www.google.com/search?q=Insta+Matik+100%2C000+BTU+Digital+Thermostat

https://www.google.com/search?q=Titanium+above+ground+pool+heater+parts

..

check gas pressure this heater overheated and improper gas will do that

R3 should be a software code on the board.
E06 is Temp Sensor but check the wires and the connections. Also remove and inspect the sensor, it may be damaged.

please refer to page 51 of this manual. http://aquacal.com/heat-pump-information/support/manuals/pdf/man-AT_HW_SQ.pdf

This explains that it is either due to your pump not pushing enough water.
Or the pressure switch is clogged or failed.
Or the wiring has shorted.

Check to make sure the circulation pump is not clogged and flowing freely.
make sure the filter is clean and flowing freely.

if this does not fix the problem, replace the pressure switch.
refer to page 56 for pressure switch identification.

This is a bad temp sensor or an open by-pass. You would have to go to your book to find these though. I'm not sure where you would find them.

you should just have to replace the casing.

Sounds like your compressor is not running. After your fan goes on in one to five minutes you should hear the compressor kick in. If it does this and then kicks out again it can be something as simple as water flow and you can adjust the valves to get more or less flow to keep it running. If not you probally need a repairman. It is ether your compressor or the capicators that start the compressor and this is all high voltage. Good luck.

The board should be 24v and you may have a problem with your transformer or the power to it.