I have an old Bryant that has very few safety switches it has a continious burn pilot. My problem is the burner will not light. I have cleaned burners, replaced thermostat. I can get the fan to run with the "Fan on" selection of the thermostat. I check the voltage at the gas vavle, from the limit switch and get none I also check voltage supplying limit switch and still none.

Have you checked the voltage coming from transformer,, should be 120 volts going in and 24 volts coming out other side,, if not,24 volts coming out,, just might need replacing

Thanks for the help but ended up being the upper limit switch just needed reseting

If your transformer doesn't have a fuse for it, I would add one. I think there are pictures of them in the sticky at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html

Adding a fuse is a cheap, easy project and saves replacing the transformer if anything goes wrong.