Help! This otherwise reliable furnace has given me trouble the last few years when relit after summer passes. I can get the pilot lit but that's it. The burner won't light. Even if I turn the thermostat all the way up. It's been thoroughly vaccumed out and I used a can of air blaster on the pilot. The last time this happened, I went under the house and tapped on a few of the gas lines and it fired up and after awhile seemed to work fine. This year though I haven't gottenn that lucky but before I call someone out I was hoping I might be able to fix it myself. There's a gas control valve and a control valve on the pilot. The thermostat has two wires and is a millivolt system.
I hope someone can help diagnose the problem and/or direct me to where I can find replacement parts for it.
I know it's a relic of a heater but I can't afford to replace or upgrade yet.
Thanks for any help!
Phil

The milivolt generator might have seen better days. If you have an electric test meter with the proper settings you could test its voltage output.

The milivolt generator might have seen better days. if you have an electric test meter with the proper settings you could test its voltage output.

Thanks hvac1000for the reply! Since I know little about this system, where is the milivolt generator located. And, since I also have no test meter, is there another way to test it. The heater does a fairly good job of heating the main house and someday a new one will need to be installed, but until then I have to make due. Would this a very expensive fix and would it be cost effective to just replace all the main parts of it?

Remember I am guessing at your problem telling you to test the milivolt generator to be sure it is bad before replacing it. Since you do not have a test tool I really know of no other way to test it.
The milivolt generator is usually right near the pilot lite and usually has two wire terminals on the ends of the wire that goes to it. This item actually generates a very low voltage signal by the use of the pilot flame and that powers the gas valve.

Some of them look like these.

Remember I am guessing at your problem telling you to test the milivolt generator to be sure it is bad before replacing it. Since you do not have a test tool I really know of no other way to test it.
The milivolt generator is usually right near the pilot lite and usually has two wire terminals on the ends of the wire that goes to it. This item actually generates a very low voltage signal by the use of the pilot flame and that powers the gas valve.

Some of them look like these.

I did some more research and it is a strong possibility that the generator may be bad. Mine resembles the picture in the lower left corner of your post. The pilot flame is blue and looks strong but the milivolt generator (right next to it) just barely flickers. I have also noticed that when the furnace is working right it is a constant blue halo.
Replacing this piece looks pretty easy but finding the correct replacement part maybe a problem due to it's age unless a universal part is available.
It's either fix the thing or go back to burning wood.:D
Thanks again for your time and help!
Phil

No problem

Back to my pilot problem, looks like a PG9 A42
Generator and the one in your pic (lower left) looks like the one. I found one online, unless you have one to sell. Is this a right port they are referring to? When the one I have is on the burner, the pilot is on the right and the genrator on the left. My generator barely flickers when the pilot is lit. I might be able to get a tester if you explain the process and how many mv I should be getting. All the wires on the gas control, pilot control and thermo look good. I also bypassed the thermostat at the wall but nothing happened, would that rule out the thermostat?
Let me know how to proceed so I don't fork out a bunch of money on something that I don't need.
Thanks a million for your clear and concise help. Sometimes us DIY'ers need some help from the pros like you! :)

would that rule out the thermostat?

Yes if you jumper it out and it did not fire over the furnace.

NOTE: the generator does not glow per-say it just generates a very low type voltage called milivoltage. It makes no difference if it is on the left side right side of the pilot since you will just be replacing the cartridge with a new one in the same location using the same pilot assembly.

Most units are rates at 750 milivolts in your applications BUT there could be different ones and since I am not there to see and measure it all I can do is give it a good guess. Most of the ones that are dime size are 750 milivolts.

The meter you need to use has to be able to read the test voltage. The test voltage does not need to be 750 but can float down to 325. The quality of the pilot has an effect on the output.

From class

As with the thermocouple, the open circuit test will tell us the potential output of the thermopile. While most thermopiles are designed to produce up to 750 mVDC, a minimum reading of 325 mVDC open circuit is generally acceptable as long as the switch is located close to the appliance and connected with properly sized wire.
The test is carried out by removing the thermopile leads from the TP/TH and TP terminals and then placing your meter leads on the two wire terminals. Once again, your reading may be negative or positive depending on polarity, but it does not matter as we are concerned with the actual number.

So it is possible to replace just the cartridge and not the whole pilot assembly? The setup looks like it does have a clip on the back of the cartridge like the one in your picture, but I didn't mess with it. Wanted to learn a bit more before I tried that. I'm asking around for a tester now with my mechanic and contractor friends and will get back to you when I do.
Thanks Teach, I'm learning a lot!
To be honest though, if I hit the lottery, this furnace will be history and a central unit with air will be a priority.

That's why you're the expert! Damn, you know your s**t sir, thanks so much for the help.
Here's what I did: Took pilot assembly off, blew everything out with compressed air, checked wires on gas control. Put it all back together and BAM! I'm back in business for now, eventually this whole set up will be replaced though. With the money I saved on this I should start a "new HVAC system fund".

Glad you got it going.