I have an American Standard Freedom 90 unit that wont fire up. The unit was installed in 1999. The thermostst signals the unit, the exhaust blower kicks on but that's it. The code is flashing 4 blinks which means "open high limit". I understand this could possibly be from the pressure switch, flame rollout switch or several other limit switches. Should I check each switch w/ a multimeter? Also, When this first occured the furnace filter was fairly dirty, replaced it but still have a problem. Could the dirty filter have cause a switch to trip which now needs to be reset?
The dirty filter may have caused the unit to cycle/open and close until the limit switch wore out and is stuck open. Should be on the firewall/panel near the burners. Usually has a L number like L170F or L250F stamped on it.
"Cut it twice and it is still too short".