Before connecting my device to the boiler, I made sure that all interfaces were working. 

There was a problem with the R1 resistor, which made the CAN bus not working, but it turned out that it was enough to correct the weld. There is working two-way communication between the two PCBs. The fuel level sensor leads have also been tested and everything is fine.

Now all that's left is programming and installation on the boiler :)