Lots of possibilities. A job for a corgi man for safety reasons. Phone Vokera for a local specialist, a decent fitter/engineer should soon sort it out.
Is the boiler pressure OK?. Do the flow microswitch and differential flue pressure switch actually operate when there is demand (check for mains voltage switching on plugs attached to PCB with a voltmeter and great care), or look for movement and click from flow microswitch; this tells you that the pump operates and water circulates and the the fan operates and air circulates through flues, and that the PCB knows this.
Check temperature controls both turned to max.
Next look for volts on the gas valve solenoid, if voltage present check resistance of windings, if open circuit call Corgi gas man to replace the valve if you are not corgi.
If no volts on gas valve despite flow and differential air pressure switch being OK check the overheat switch is OK (closed circuit) and that the thermister is working (should not be open circuit, but thousands of Ohms cant remember exact room temperature value), if thermistor faulty boiler may think water too hot and not fire up.
If you find the differential flue pressure switch not operating check silicone tubes put on correct spigots and not swapped over after cleaning venturi. You should be able to operate APS by blowing/sucking on a tube going to the switch to verify switch not bust.
If the PCB is getting the correct signals and still not operating gas valve PCB may be faulty. There is a fuse, check it is intact too (it should never blow). If you know electronics check for dry joints around power transistor legs, shorted full wave rectifier diodes , blown power transistor etc.
This work requires the ability to read the manual circuit schematic, an understanding of boiler operation and fiddly probing onto the plugs that go to the PCB with mains voltages present. If possible use an earth leakage circuit breaker or an isolating transformer on mains supply. It is potentially dangerous as water, mains voltage and gas are present!.
If you have any doubts about competence then get a Vokera Corgi specialist on the job. Phone them (contact no on website). It may be illegal these days to work on your gas appliance if you are not a competent person, and if something goes wrong the argument would be that you were not competent.
c
January 2008
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