York - GM9S100C20DH11G
A few days ago this started acting up. Would not keep temp set on thermostat (heat). Thermostat not the problem. Furnace runs but seems to shut off after 5-10 minutes for unknown reason. Control PCB LED flashes 4 RED which means Limit SW. I replaced Limit SW but that made no difference. Same result. Also cleaned Flame Sensor Rod. Burner, igniter, blower everything else seems to be working fine, just won't keep running. Goes through about 4-6 cycles of burner starting but then shuts down. Only LED indication is 4 Red flashes. Manual says could -
"*-* RED - This indicates that a primary or auxiliary limit switch
has opened its normally closed contacts. With this fault code the control
will operate the supply air blower and inducer. This condition may be
caused - dirty filter, improperly sized duct system, incorrect blower
speed setting, incorrect firing rate or faulty blower motor. "*/-*
I cleaned the vent grills 4 months ago, but pulled them out now anyway. Same problem. Blower is running normal. Have no idea what "Incorrect Firing Rate" is.
Also, I hear a clicking "CLICK-CLICK-CLICK-CLICK" right before I get the 4 Red flashes on the status LED. Sounds like it is coming from the Gas solenoid. I am wondering if perhaps a relay went bad on the control PCB.
After it shuts down, 4 Red flash status remains. Thermostat still calling for heat. I sw the thermostat to Off, then back to Auto, and the Furnace starts back up again and cycle/problem repeats.
I'm stumped. What does it sound like to you? Any ideas?