baxi 105e safety thermostat fault light?

my baxi 105e combi boiler is desplaying the safety thermostat fault light.
when there is HW or CH demand the flame ignights but HW is at best tepid, temp lights show while there is demand but return to fault light once demand stops.

reset does not turn out the light, power of does but returns once demand has again cesed.
tested the thermostat there is continuity accross the terminals.
bridged conector to safety stat, problem is unaffected so the stat is not the problem

thanks for reading anyone got any ideas?