Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Selector switch set to DHW/CH
Preheat switched OFF
CH temperature set to MAX
Remove external controls connection to boiler
(pin 7 main terminal block) and leave in a safe position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Is C still displayed?
Note.
1. Image shows appliance with no external controls fitted
(external controls linked out).
2. Connection 1 of terminal block is far left when facing appliance front.