Before the burner gas valve opens the boiler has to prove water flow through the heat exchanger and proper air flow through the flue.
A pin is pushed by a diaphragm through a water tight sealing 'o'ring that operates a microswitch to tell the boiler that there is water being pumped around. Check that you can hear the water pump working, and look for the microswitch actuating pin moving.
If the pump hums and the pin moves the check the microswitch; this part (as well as the domestic hot water demand microswitch) seems prone to failure. Disconnect the boiler from the mains electricity and verify microswitch operation by checking with a multimeter having unplugged the microswitch connector from its socket on the control unit first (or compare the clicking noise it makes with the clicking noise of the DHW microswitch for a rough idea).
Faulty flue flow detection is often caused by dirt on the flue venturi, and starts of as hot-cold water cycling for domestic hot water. Google 'vokera 80sp cycling' or somesuch to find information. This does not sound like your problem. (It is easily spotted as the boiler goes from full flame to pilot with a click, rather than modulating down, with severe DHW temperature changes. Awful for showers or baths, but not a problem for central heating.)
Get someone who knows this specific make and model to fix it rather than pay a corgi generalist to slowly figure it out(or not) at your expense. Phone Vokera for a local repairer.
If you think that you are competent to fix it then you will need a copy of the manual. The manual can be downloaded from the vokera website (enter as tradesman, literature, old models). This boiler needs servicing every year or so to keep it running nicely. Servicing should include fitting kit t0019 and doing various pressure tests as well as a clean. A new microswitch should cost around a tenner.
cd
February 2007
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