Hi the same problem has been plaguing us too.
First of all replacement of the Thermistor did the trick £8.80
But problem came back after 6 months.
Its not the Overheat thermo approx £6
These are both easy to replace and test. Thermistor pull out on plastic clip.
The overheat sensor unscrew with a spammer. fiddly dependant upon how the boiler is installed as reall need space to lefthand side, or suitable box spanner.
The guy at the spares counter said its the worst boiler known to them for problems with the PCB and often is the resistors on the board apart from dry joints.
The Resistors are 6 band types. Anybody help with the values of R33, R34, R37, R38, R84, R85, R87, R88, R41 and R42?
They appear all to be the same value but colours are difficult to ascertain
Most are wired in series. BUT on my board the resistance of R37 and R38 is quite different from rest and this appear to be wired to one lead of Temperature Thermistor.
Any help would be appreciated and also where replacement resistors can be sourced from.
Finally one other thing. If you have retro installed TRV's then unless you have a Automatic Pump Bypass valve fitted (Now mandatory on new installations under Part L of the new Building Regulations) this may also cause Lockout.
It seems although the Installation Instructions for the Suprima say that 1" (28mm) Flow and Return pipework is required to Pump and a Bypass of at least 1.6m if the CH and HW circuits are controlled by Motorised Valves. Often the bypass is a 15mm constant circuit to the Bathroom Rad rather than 22mm. 15mm is not as recommended - Also this rad MUST have fully open lockshields on it to work. Yes it seems to work most of the time, but is not as recommended.