Thanks for all the tips I've picked up over the past few months... Time to actually make a post.
Out of the blue our EM5800 has started pouring water directly out the overflow (via the diffuser) instead of pressurising and coming out the steam wand or group head. I've seen comments in other threads about clogged 3-way valves but elsewhere it's been stated that the EM5800 doesn't have one (backing that up, the manual certainly doesn't say anything about back-flushing, I don't recall any obvious dumping of pressure at the end of a pour, and the puck ends up with a pool of muddy liquid on top).
However, if it doesn't have one, then what is the PCB-controlled solenoid valve that has one hose connection to thermoblock, one forwards to steam wand and/or head (can't get enough access to see) and one down to diffuser and overflow? It looks to be this latter connection that's draining when it shouldn't.
It happened quite suddenly - machine worked ok this morning but not since this afternoon.
De-scaled a few months ago (and the water here is pretty soft), and have given the group head, screen, seal and collar a good clean today to no effect.
What should I try next? Thanks!