My furnance blinks 4 times, indicating the Max and Aux limit switch are open
On the limits there may be a button in the center of the limit that you can depress, if it is tripped it will click and then work, the main limit is automatically reset unless it is bad, that is rare, if it will not reset it is bad.
SOURCE: Goodman GMP125-5 limit switch open
There may be as many as 6 limit switches on the furnace. The first thing to do is turn off the power to the furnace. Look around the burner box. You will see possibly 3 small maybe brown plastic swithes with two wires to each and a button between the wires. Press the button on each one. If it clicks that's the one tripped. There may also be the same kind of switch(s) in the blower compartment. There's a self resetting limit on the middle of the furnace it also has two wires but looks like a black or dark brown plate about 2 1/2 X 3 1/2 inches. You have to check that one with an ohm meter. If any of these switches are tripped there is a reason. Check your filter and overall air flow. with low airflow the furnace may overheat. The switches around the burner box are roll out switches. Watch to see if flames are coming out of the burner area. especially when the blower comes on. DO NOT bypass any of these switches. Call a certified tech. to check it out.
SOURCE: Limit Switch
It is rare that a limit switch goes bad. It is a safety control. If it is shutting off your furnace there is a problem that it is protecting you from.
If the limit switch is shutting the unit down your furnace is overheating. Check the filters. Check for any obstruction in the blower assembly. Check to see if you can raise the blower speed. Make sure all registers are open in the house. Your evaporator coil could be obstructed if you do not take good care of your filters.
SOURCE: 4 blinks, main limit switch open or aux limit
There is a limit switch above the burner with 2 wires going to it. Take a jump wire and jump out the switch to test it, It is 110 volts so be careful. If it fires, that switch is bad and will need to be replaced. There may be up to two more switches on the same circuit. These switches may be reset-able if there is a small dark red button between the two terminals. Why did the unit overheat in the first place. Is your air filter clean? Is the room air blower working? Are all return air grills free of obstructions?
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