The display panel on a Goodman furnace has a built-in trouble-code that uses successive flashing LED lights to indicate problems. However, even with the lights, you must troubleshoot the furnace to determine the nature of the issue. For example, three consecutive flashing lights on the Goodman furnace means there is an issue with the pressure switch circuitry. You must troubleshoot the switch to determine the reason why it is malfunctioning.
Disconnect the electrical power from the furnace by turning it off at the breaker panel.
Remove the screws that secure the front panel onto the furnace and then place the panel off to the side.
Locate the pressure switch tubing that connects to the induced draft blower on the furnace. The tubing is black and flexible.
Pull the tubing off the blower and then trace the tubing to the pressure switch. Remove the tubing from the pressure switch.
Blow the tubing out to clear any debris trapped inside the tube and then reconnect the tubing to the switch and to the blower.
Inspect the electrical wires that come from the switch for any poor connections. If the wires do not securely plug into the switch, press them in until they connect all the way.
Turn on the furnace and wait for it to run. If the lights come back, the pressure switch is defective.