tocateclas BUT with the steam boiler hot, the machine fails. No matter if it’s the first espresso of the day, the second or the last one. Believe me, I have tried everything and the only fact that remains the same is the different behaviour depending on service boiler temperature.
Is the brew water preheat system on when this happens (HX on setting). As this could be the only possibility, not certainty that back pressure from the HX tube may make the PID behave oddly with respect to pressure. Although even at 130C only around 1.7 bar is the push force required to overcome it (possibly more if the expansion of that very small amount of cold water in the HX tube causes a higher pressure momentarily). The pump would then move to a high pressure briefly to overcome it.
HX on is a setting for cafes, not home use, so it should never be used otherwise.. With HX on, the way to the group closes after the shot, and the systyem is sealed again.
HX off closes the route via the HX tube and water routes around the service boiler to the group. The only way I can think that a similar issue “might” happen as with HX on is if the solenoid that prevents water entering the HX tube is leaking back slightly and raising the pressure in the other side of the system… in between any 1 way valve and the solenoid leading to the brew boiler.
The lower temp of 120C, may be insufficient to raise the pressure enough to cause the issue?
All guesswork?