15
Call for heat,
t-stat closes R-W
Power On
L1 A/C Voltage
No
Yes
Yes
Power up
Selector Switch
Check incoming
power supply to
furnace and
selector switch
No
Check voltage
at WxC
Power at gas
valve
Check circuit from secondary
voltage through all safety switches
to gas valve
Yes
No
No
Gas valve opens
first stage, low fire
Pilot lit
Relight pilot.
Consult "Pilot Outage
Problems" in service
manual
Yes
Yes
24 Volts at Gas Valve
Lever in ON Position
Inlet gas pressure OK
Replace Valve
Yes
Yes
14 second
second stage
high fire
No
Yes
Yes
Air Circulating
Blower Starts
No
Yes
Thermostat
Satisfies
No
No
Fan Switch
closes at 115˚
Power at
Motor
Yes
Replace
Motor
No
Replace
Switch
Open R to W
Yes
Shut down,
Replace Thermostat
Air Circulating
Blower Shuts Down
Yes
Stand by
for next cycle
Fan Switch
opens at 95˚
No
Replace
Switch
Troubleshooting Sequence – M1G 056-070