Another boiler fault diagnosis needed

Hi,
My Ariston Microgenus 27 (combi) has developed an annoying fault. Sometimes when it goes to fire (either to heat the rads or do the DHW) it doesn't "catch", and instead a red fault light appears and everything remains cold. Pressing the reset button next to the red light causes it to try again and usually (but not always) it fires the second time round. This is particularly irritating when you're standing naked by the shower waiting for the stream of water to warm up, and the realisation dawns that it never will unless you go running down two flights of stairs to press a button.
This is probably one for the engineer, but it would be useful to have some idea of what's up before I call him out.
One other symptom which may help the experts is that whenever the boiler does fire up, there is quite an explosive noise from the (balanced) flue (I'm using the word explosive loosely here), in other words there's quite a whoosh. I seem to remember reading somewhere that this tends to happen when you have a long flue, although that doesn't apply in my case - the boiler is on an outside wall and the flue goes straight through it.
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Sounds like the spark is weak - has it been serviced (cleaned) recently?
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On Wed, 30 Aug 2006 15:16:33 -0700, Martin Pentreath wrote:

This does sound like an intermittent ignition failure. So check it out. Leads, shorts, electrodes, debris and dirt near the electrode...
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Ed Sirett - Property maintainer and registered gas fitter.
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