I have an older Rheem gas furnace, Model RGAA 150C. Tried to turn on the heat this morning without luck. Tried to eliminate the thermostat first as the culprit. Disconnected the thermostat and used a jumper between the red wire and the fan; and between the red and the air conditioner. Both worked fine. The pilot light is lit, but nothing happens when I jump between the red and the white wires. Placed an ohmmeter on the wires coming from the limit control - schematic shows normally closed contacts, and I got good continuity there. Also showing continuity (+/- 1 ohm) across the terminals on the gas valve, A Robertshaw 7000 (with wired disconnected). Starting to suspect the relay at this point, but I'm not sure what I'm looking for when I troubleshoot it. I currently have the thermostat wires jumpered to give me heat, and I'm not really reading any voltage at all. No work has been done on this system since last winter when it was working fine. Sure could use any help that can be offered.

Update - With power on and the fan on "on" I jumped from the thermostat fan wire to the gas valve - it cycled the heat on. Back to the white wire from the thermostat - nothing. A quick continuity test on the white wire from each terminal end ( thermostat to furnace) showed an open circuit. Pulled the wire and sure enough found a clean break in my white wire. In the process of fixing it now, but I'm confident that my problem is solved. If not, I'll post again. Thanks to all who, in other related posts, gave me enough insight to successfully troubleshoot this system. Ain't the internet awesome?