Combi boiler Hot water issue

[Marty1966]

Hello
Ongoing problem with a Combi boiler (Remeha Avanta Plus 28c)
When hot tap is opened boiler starts up and hot water runs for about 15 seconds then runs cold and boiler stops. Interestingly the CH Flow pipe is absolutely red hot despite no heating called for.

When calling for CH it locks out.
The following parts have been replaced but problem persists -

PRV

DIVERTER VALVE

PLATE HEAT EXCHANGER

HYDRO BLOCKS L&R

HYDRO BLOCK FILTER - L

HYDRO BLOCK VALVE - R

DIVERTER VALVE MOTOR

PIPING SET HYDRAULIC

FLOW SWITCH ASSEMBLY

Short of the main heat exchanger and the pump what else could be the issue? Thermostat?
On the surface it still points towards the diverter valve which I can replace again but highly unlikely with a new one just fitted …
Any thoughts appreciated

 

[CountryFan]

Isolate the flow valve and see if it stays on. Possibly crossed pipework?

 

[Marty1966]

The pipework hasn’t changed from when the boiler was working perfectly

 

[Marty1966]

I’ve tried with the flow valve open and closed with the demand for HW and the result is still the same

 

[CountryFan]

Edit.