Fault code A9 on Vitodens 50-W boiler

redvee

On my Vitodens 50-W boiler, I am getting an error message 'A9' which according to the manual means - 'Sensor inputs are not read correctly'
The boiler is less than two years old, but the house thermostats are from the original build, so probably 15 years old or so.

When I re-set the boiler, it will fire up and heat the water for the hot taps, but the water is not being circulated through the radiators.
There are two wired in thermostats (upstairs and downstairs) - these connect to myson motorised valves which separately control the hot water flow to the upstairs and downstairs radiators. There is a third myson motorised valve which controls the hot water supply.
I can remove the motorised valves and leave the valve in the open position, but the water is still not circulating - I'm assuming that the boiler isn't being 'told' to run the pump to circulate the water to the radiators.

I'm guessing that the most likely problem is with the thermostats, but looking for advice on trouble shooting, or how to start fault finding.
Is it worth replacing the thermostats as a start ? Any recommendations as to type of replacements?

We have a Climote hub that was put in (as a freebie) which basically just acts as a remote time clock for the boiler - it's not connected to the thermostats - this was the setup advised by the Climote installer when he set it up.
Any thoughts and experience would be greatly appreciated,
Thanks in advance.