Zanussi gas hob won't stay lit

CGORST

Hi all
I have got a freestanding Zanussi gas cooker.  None of the 4 gas burners will stay lit.  They light ok but when I let the knob go, the gas goes out.  The only odd thing is that if I light the small oven/grill directly below the hob and leave it to warm up, the gas burners stay lit!  I know with it being a gas cooker I can't fix it myself but I'd rather have an idea of what the problem might be before I try and get it repaired.  If anybody has any ideas, please let me know - it's driving me mad!  

macman

Possibly the FSD is faulty, but impossible to be sure.

Sandtree

Is this hob new to you? Has the problem just started? If you hold the knob in longer will it stay alight

CGORST

Well the problem has been intermittent.  Sometimes it stayed lit and other times it didn't, sometimes some of the burners worked while others didn't but it has recently worsened.  If I keep the knob pressed in, the flame stays lit but as soon as I let it go, the flame goes out.  The cooker isn't new - it's probably 6-7 years old.  

frogglet

I've had gas cookers do this. Id have to hold the knob in for a long time. Have you tried cleaning everything.
I think its to do with the thermocouple thing which needs to heat up.

Ebe_Scrooge

I agree with the previous posters - it sounds very much like the FSD (flame safety device), which is a thermocouple sensor.  Basically, it's designed to cut off the gas if the flame gets blown out, to prevent unburnt gas escaping.  It's a very simple device that just reacts to heat.  The fact that lighting the grill underneath would tend to support this theory - though it is a bit unusual for all four to be faulty, you'd normally get just one burner playing up.

Make sure everything is cleaned, and also check that the sensor probe is actually within the flame when the burner is lit.  If you look closely, you should be able to identify the probe pretty easily.  If it's been knocked out of place, that means it doesn't get heated by the flame, so it thinks the flame has gone out.  If this is the case, you can try very gently easing it back into the correct position.

Sandtree

CGORST said:

Well the problem has been intermittent.  Sometimes it stayed lit and other times it didn't, sometimes some of the burners worked while others didn't but it has recently worsened.  If I keep the knob pressed in, the flame stays lit but as soon as I let it go, the flame goes out.  The cooker isn't new - it's probably 6-7 years old.  

When you press the knob in you, in laymens terms, bypass the flame safety device (FSD) hence you can ignite the gas and in theory the sensor then gets hot and keeps the gas valve open when you release the knob. In theory if the flame goes out the FSD cools and so shuts the gas to ensure you dont have a gas escape however a faulty FSD or one thats out of alignment wont get hot (or doesnt react correctly from getting hot) and so cuts the gas as it thinks the flame is out.

I'd be surprised that the grill produces enough stray heat to make a material difference to the FSD but guess its possible if not insulated well and the FSD is getting almost all the heat it needs to keep the valve open from the ring. 

Would agree with the others in checking everything is clean and that the probe is correctly positioned in the flame to get heated. Also check that the flame is forming a full circle as uneven release of the gas from the hob can also cause a problem if the probe is in an area of low gas... though to luck out and have the same problem with all 4 seems unlikely.

knightstyle

We had a similar problem some time ago.  Try lighting one gas ring and while holding the button down use a long spout lighter to heat the probe till it glows, then see if the ring stays on.  The probes and electric bits associated with them are easy to replace DIY but disconnect from the mains first.