F5 in the Glow-worm fault list is 'overheat'. As with all overheat failures it is a symptom and the cause must be diagnosed, never just press reset and walk away.
With open vented boilers over-heat can be for many reasons, normally external to the product itself such as:
Air locks in the system
No (or lack of) water in the system
No circulation (pump issues)
The pump not wired correctly back to the boiler (preventing pump over-run to cool the boiler after shut down).
Basic investigation of these areas should normally reveal the reason for the over-heat. If this is not obvious, investigation further into water flow (pump speed and temperature differential), water supply (correct cold feed and open vent configuration that is not blocked by debris) and correct wiring of the pump and controls will reveal the reason.
It is generally the exeption that the thermisters are actually faulty or the PCB is not reading them correctly as this is picked up with other fault codes. These can be checked by a multimeter if necessary and compare the value in ohms to the displayed temperature (there is a table to compare this).
Remember, to work on a Gas Appliance you must be declared as competent to do so by Gas Safe Register (In NI and Channel Islands Corgi registration still applies). Working on a gas appliance means that safety checks need to be performed to ensure its safe operation.
David Parker - Glow-worm (Product Manager), July 2009
