Vaillant VCW GB 221 H Cutting out after a few seconds

[mdskids]

Hi there. Just joined the forum looking for advice about our ageing Vaillant Combi (VCW GB 221 H)

Recently we've had trouble with the pilot light going out and being very difficult to relight, so I changed the thermocouple, and it now seems to stay on fine. However...

Now, when the boiler fires up (for either CH or DHW) it burns for a few seconds, and then there is a loud click from the gas section/operator area and the gas (and the pilot light) shuts off.

I've tried to follow the troubleshooting advice in the manual, and this is what I've already checked.

Green LED is lighting on the control box, when CH/DHW is on (even once gas has shut off)
Pump is turning fine when CH/DHW is on
Diverter is moving fine to activate microswitch
DHW thermostat/safety switch has been checked for continuity (contacts open)
CH thermostat/safety switch has been checked for continuity (contacts closed)
Main heat exchanger thermostat/safety switch has been checked (contacts closed)
Disconnecting NTC resistor makes no difference - still fires up for a few seconds and then shuts off
When DHW/CH is on, I get 20V on the operator contacts, and I'm still getting 20V even after the gas has shut itself off (until DHW/CH is turned off) - this suggests to me that the PCBs are fine?
Just to double-check it wasn't a blockage/overheating issue, I've tried temporarily overiding (by disconnecting/shorting) each of the thermostats, and it still cuts out after a few seconds.

Because I'm getting 20V on the operator but its still shutting off, does this mean its a gas supply/pressure thing?? I don't have a gauge to measure any pressures, and anyway - messing around with the gas section is beyond my DIY capabilities.
BTW, this problem is a new one - we didn't have this shutting off issue before the pilot light problem/thermocouple change.

Is there anything obvious I've missed/not checked before I concede defeat and get a pro in? I'm reluctant because I don't think there are any engineers nearby any more who know this boiler well.

PS Confession time - the boiler has not had a proper service (other than a bit of a hoover out) for years

 

[gas112]

Its a simple fault that a basic service will fix

 

[mdskids]

Thanks for the quick reply!

That's reassuring, but could you elaborate - what do you suspect is causing the fault?

 

[gas112]

dust

 

[mdskids]

You, sir (..or madam?!?) are a saint

Decided to service it myself, as per the manual's instructions.

All burners and heat exchanger brushed and hoovered, probably cleaner inside now than it has been in at least a decade

et viola! Now works perfectly - fires up nicely, pilot light looking healthier. Not only that, but it's running quieter and hotter (and, I assume, more efficiently).

Now my only problem is to shift that incredibly stiff DHW temperature control knob down a touch!

Many thanks gas112!

 

[ScottishGasMan]

You, sir (..or madam?!?) are a saint

Decided to service it myself, as per the manual's instructions.

All burners and heat exchanger brushed and hoovered, probably cleaner inside now than it has been in at least a decade

et viola! Now works perfectly - fires up nicely, pilot light looking healthier. Not only that, but it's running quieter and hotter (and, I assume, more efficiently).

Now my only problem is to shift that incredibly stiff DHW temperature control knob down a touch!

Many thanks gas112!

What was the working pressure? Burner pressure and emmissions radings from the boiler?

assuming your fully qualified to do what you have done and know how to check that you havent left it unsafe?