I had problems with my bolier, which doesn't start. After some investigation the ignition control box SIT 503 EFD is faultily. The electricity was present, but the ingnition process failed.
I removed control box SIT 503 EFD from the boiler and open it with aim to try find the source of undependable function.
The failing was caused by relay NAIS JS1-48V-F-H77. (On the picture it is pointed by screwdriver). Resistance between contats 1-3 or 1-4 should be up to 1 ohm.
I replaced this relay with Panasonic JS1-48V, which has same dimensions and parameters. Original NAIS JS1-48V (on the left side) and Panasonic JS1-48V (both are Matsushita).
After some soldering it was possible to put the ignition control box SIT 503 EFD back to bolier, which works fine.