Failing diverter valve?

[Albumen]

I've got a (fairly old 2005 I think) Vaillant Ecotec Plus. The central heating seems to be on more than it should. I've checked communication between the (wireless) thermostat and the receiver, and that seems to be working OK. I turned the thermostat down to "off". There's one radiator that has no TRV, which I believe is how the system is meant to be set up? Anyway, that still gets hot. So I investigated. The flashing C light is appearing a lot. I think that's the "comfort" DHW preheater, yes? Could it it be that the diverter valve is failing and so some heat is being sent to the CH system? DHW is still hot, by the way.

I've had the diverter valve fail before, but then it was more binary - no DHW when asked for, but the Central Heating came on.

By the way, this was serviced only a couple of weeks ago, by Vaillant. The engineer declared all fine. It's worth pointing out that though this is an old boiler, most of the components have been replaced, due to a rather strange thing with Homeserve a few years ago. The main burner, circuit board, pump, gas valve are all new. And maybe some other parts too. One engineer said - "you've more or less got a new boiler, it's like your grandfathers axe".

UPDATE. So, I've managed to turn Comfort Mode off. I let everything cool down. Luckily there's a sink, with a hot tap, right next to the boiler. I held my hand on the left most pipe (which I think is flow?) and turned the hot tap on. Sure enough, the flow pipe heated up immediately.

So, I'm guessing that it's the diverter valve?

 

[CBW]

Is this the combi boiler version or system boiler version?

 

[Albumen]

Is this the combi boiler version or system boiler version?

Combi, hence the DHW.

 

[CBW]

Likely the diverter, could be the harness if any wires trapped. Could also be blocked plate heat exchanger and or canoe filter.