I've got a GM Delco furnace model 999 that won't produce heat. The pilot flame is working. I don't see any loose or broken wires. The thermostat clicks when turned on. Is this a DIY or time to replace it. Thanks

You need to check the contacts on the gas valve for 24 volts AC. If not, it should be a fairly easy DIY to work back to the transformer and replace what isn't working. I have never messed much with a gas valve, and I am not sure I should tell you to.

If the furnace has a pilot light, it could be an old, less efficient one with a bad, or about to go bad heat excanger.

To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.

It may be your thermostat. Pull cover off t-stat & jump a wire across your red & white wire. Also check the gas valve to make sure its in the on position.