I have a very old AFCO gas furnace (@ 1950) - this is the first problem we've encountered in 10 years - it has a standing pilot which is connected to the gas line before the regulator and valve - the pilot also has 2 wires that act as a switch to the gas valve - everything is fine except for the valve does not open when the thermostat calls for heat.
I can manually open the valve by pushing it open from underneath -it has a spring loaded , push in and turn 'button' on the bottom. Then I have to release it to turn it off - the pilot has a ceramic(?) sensor attached to it

Is there a way to test either part before buying a new valve? - thanks

You may have to to either clean the thermocouple or replace it. The thermocouple detects if the pilot is on. If it is on and heats up the thermocouple then this allows the main gas valve to open. If the pilot is out or the thermocouple is damaged then the main gas valve will not come on. This is a safety feature.

Your description does not sound like the conventional tubing like thermocouple. If it was, they are cheap enough and easy enough to find and replace to replace it as the first step. If not, check to see if either the pilot light or gas valve gives a voltage rating. If so, see if you have the voltage indicated on the wires from the thermocouple. If not, replace the thermocouple. If so, replace the gas valve. I have never seen anything that looks like your description. You may have to replace the whole works with a more conventional one if you can't find the parts you need.

Thanks for the replies - I went to the most recommended local supply house and they couldn't find any replacement parts on the market for the pilot sensor - the original unit doesn't have a traditional thremocouple - so I am replacing the valve and pilot to a more modern version -
It turns out that the sensor, if available, would have probably cost more than the new valve and thermocouple.
So now I'm off to get the needed pipes to make the switch because, of course, the new one is not exactly the same size as the old one.

Thanks again for the replies

If this is the older style pilot sensor the wires inside the small metal box may have come loose. Reconnect the wire(s) and the pilot sensor should work.

Is this a 12 or 24 volt unit?

Yes Lotta it is an older style - or was now - I replaced everything today - I checked the wire connections and then I bypassed the sensor and the valve fired right up -
Once I got it out I realised it was pretty far gone - the contacts were missing some of the ceramic and were very dirty and brittle - I couldn't find a replacement so we went with a new valve and a thermocouple - been working fine for 5 hours now - woohoo!! :cool:

Glad that it worked out for you. Stay warm.