With HW revision board 1.5, here are some finding in looking for the working of the C245 on this S99.
The board 1.5 is different from the 1.4.
It seems that there's only 2 "real" connexions going to the C245 tip: the center and the little right.
The big part of the tip seems to go only to the ground screw: this part must be the common part of the Heater and Thermocouple.
After cutting the trace to the amplifier, I have soldered a wire and checked alternately the screen to see what T° it shows.
I have named 1,2,3 the three tabs of the C245-B1 clone I have had with my Sequre (like on the 1rst photo).
Table of the reading of the Thermocouple(TC) value I have got from the S99 Oled screen:
+ -
TC 2 3 : 19°C
TC 3 2 : 19°C
TC 1 2 : 19°C
TC 2 1 : 19°C
TC 1 3 : 10°C ADC value (in the Calib menu) stays solid at "1" without moving
TC 3 1 : 10°C ADC value (in the Calib menu) stays solid at "1" without moving
The mesured R value is (with a Fnirsi HRM-10 Internal Resistance Meter):
+ - + -
R 1 2 (or 2 1) : 5.32 Ohms
R 1 3 (or 3 1) : 0.035 Ohms
R 2 3 (or 3 2) : 5.34 Ohms
With my DMM on T° mode, and hotair gun set at 100°C on the tip:
I find the (+) of the Thermocouple on pin 3, and the (-) is either 1, or 2.
All that is strange, and not like some pinout I have found for C245 tips

I have to dig all that, don't know if anyone could help ?
Edit: After drawing some draft on paper with all these data, I think I have found how the tip is connected to the board

But again, I don't understand how the Thermocouple is read by the MCU because of the strange reading between 1 and 3 by the S99. Will check that later