Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Gain access to wiring connections for the
TR2 room stat and programmer PCB

Measurement:

Confirm wiring between the boiler programmer module and
TR2 room stat is correct, in good condition and routed correctly?
Wiring connections should be as follows;
Terminal 3 on TR2 connected to terminal 3 on programmer.
Terminal 4 on TR2 connected to terminal 4 on programmer.
Terminal F on TR2 connected to terminal F on programmer.
The cable should not be routed close to other electrical cables, this can cause inductive interference (minimum separation 100mm).