Ravenheat RSF820/20T Not Firing Up

[Poisonata]

I have a Ravenheat RSF820/20T combination boiler which has stopped firing up on both hot water and central heating demands.
This happened before many years ago and in this instance it was a small blockage in the pipe leading from the main fan unit to a microswitch which clicks in when high speed fan mode is active.
The system pressure is 1 bar, the pilot light is on and burning correctly so my thoughts are turning to possible thermocouple failure.
I'm fairly sure it's the thermocouple as when you blow out the pilot light and wait for it to cool, normally after a minute or so you hear a loud click as the auto gas cut off kicks in. This I can no longer hear.
I have two questions:
First is there anything else I could check which I haven't mentioned and second is there a way to actually check the thermocouple isn't functioning using a volt meter or other device.
I am fairly adept with the boiler and have replaced many parts in the past. Where my skill is lacking is in the diagnosis of the problem.
My thanks in advance for any replies.

 

[fazedup71]

your not going to get much help on this one, best advice I can give you is get a registered installer in to fix the problem. You can check the thermocouple with a multimeter, should be getting around 30mv, but you should not be changing it yourself.

 

[Poisonata]

Update:
I removed and cleaned the thermocouple, the end (bit that goes in the flame) looked well oxidized. After a minute with some wet and dry paper I replaced it and now the boiler is firing up just fine.

@fazedup71, thanks for the information regarding the reading the thermocouple should produce. I understand why registered gas fitters would not want to respond on the grounds of safety and the fact there is little in it for them (other than a thank you).

It's interesting how it seems a small degree of corrosion on the end of the thermocouple is enough to stop it functioning correctly, I would be interested in knowing the exact mechanics of how it worked and why this is an issue.

All's well that ends well.

 

[mr-crs]

The fact that the pilot light is lit suggests the thermocouple is working correctly, that being the case it would seem your fault lies elsewhere and could reappear. When was it last serviced? It may be wise to have a service carried out by an engineer who is familiar with this boiler before
you start buying unnecessary parts should it fail again.

 

[commercialman]

I'm fairly sure it's the thermocouple as when you blow out the pilot light and wait for it to cool, normally after a minute or so you hear a loud click as the auto gas cut off kicks in. This I can no longer hear.
.

If this were the case how did the pilot manage to stay alight? (let by)

Sticking fan can cause the pilot to extinguish (low speed scavenging) , can also cause the build up of oxides on TC tip.

Have a registered gas engineer take a look.