Many Goodman domestic furnaces installed from the 1980s to the present time are the 80 percent efficient Janitrol Type GMP Models. These include model numbers 050, 075, 100 and 125. When the furnace stops working, a trouble code LED is visible through a small round sight glass mounted on the blower access door. The number of flashes allows you to interpret the codes to give you an idea of the maintenance required. Always remember that gas and electricity are both dangerous commodities. With one or two exceptions, a certified Goodman technician must service the furnace.
Before Calling a Technician
If the furnace does not start, check the following before scheduling a service call: Verify the thermostat is turned up. If that doesn't work, identify the furnace breaker switch on your home's electrical panel to ensure it's in the "On" position. Also make sure the shutoff switch for the furnace is on; the switch may be mounted on a wall near the furnace or attached to the side of the furnace. Look for a roughly 14-by-20-inch pleated filter mounted somewhere near the furnace. If the visible ribbed surface is clogged, turn the furnace off and clean off the dust with a vacuum cleaner. Restore power. If the furnace still fails to start, call your service technician.
One on-and-off blinking flash means that the furnace has locked out after three failed ignition attempts. Either shut off the power to the furnace for 20 seconds or lower the thermostat so that the furnace does not try to heat up, then reset the thermostat. If the furnace fails to start, wait one hour to allow it to automatically reset itself. If that doesn't work, schedule a service call. Five bulb flashes tell you that your furnace senses a flame without a call for heat. This could be from a gas valve's slow closing causing the burner flame to linger. Wait 15 minutes. If the flashes stop and the furnace heats up, the fault has corrected itself. If not, or if the furnace tries to start and the light keeps on blinking, the furnace requires professional service.
Service Call Required
Two flashes indicate the draft blower supplying combustion air to the furnace is not working or your furnace has a short in the pressure switch safety circuit; bellows-operated pressure switches turn on when the combustion blower supplies enough air, and off when it senses too much pressure. Three flashes reveal that your furnace has an open pressure switch circuit or it has an induced draft blower operating prematurely. The second blower helps overcome backpressure in the combustion chamber. Four flashes of the light mean your furnace has a primary limit circuit open; all limit switches shut the furnace down in the event of dangerous conditions occurring. Contact your technician to fix these issues.
Seven light flashes are a warning of a low flame sensor micro-amp signal; the flame sensor tells the control module when a satisfactory burner flame has been established. Eight LED flashes identify a bad igniter; the igniter starts the flame much like a spark from a disposable butane lighter. Nine flashes identify a bad high-stage pressure switch indicating excess airflow from one of the blowers. Goodman type GMP domestic furnaces do not have a trouble code that flashes six times. But a continuously flashing light means the polarity of the 115-volt circuit has reversed itself due to a bad AC adapter or a spike in the mains supply. Schedule a service call from a qualified Goodman furnace technician or a certified electrician to correct the polarity problem.