I have a ducane furnace model MPG075B3A. The gas valve is flashing 4 flashes the code is limit or rollout open. I cannot find my owners manual to find the location of these switches. Any help would be appreciated.

Look on the inside of the blower access door for a wiring diagram.

I have the same problem and I looked but still can't figure out how to manually open the limit switch

Is it a Honeywell smart(dumb) valve by any chance? Note you normally do not open the limit switch on newer furnaces. Post some pictures so we can see what you have.