Main limit switch

Dont really understand .Is it a furnace with electric heat ,if it is then Check to see if contactor or relay is stuck closed ,also you heating element should not get so hot there should be a limit switch to shut heater off .Maybe bad limit and bad relay .Your electric heat should shut off when fan shuts off

It sound like the limit switch is shutting off and not coming back in to play .

Hello, a limit switch will open when the furnace gets too hot, the switch senses that the temperature is too hot and shut down the furnace for safety. Uusally a limit switch will open because of lack of airflow across the heat exchanger, this could be due to a dirty air filter, dirty blower wheel, or the blower motor itself is not running at the proper speed. I hope this helps.

Heating Mode (Goodman Service Manual)

· The furnace control checks for an open main limit (this
limit is normally closed). If the limit is open, the furnace
will remain inoperable until the limit is closed. During
an open limit the circulating air blower will be
energized. The status light will blink four (4) times.
· The room thermostat reacts to a demand for heat.

· The control will then check to insure that the vent
pressure switch is open. If, at this point, the vent
pressure switch is closed the control will blink two (2)
times and will remain inoperable until this situation is
corrected.

· The venter blower is energized.· The vent pressure switch will close when it detects a
pressure in excess of its setting. If the pressure switch
fails to close the status light will flash three (3) times.

The sequence cannot continue until the pressure switch
closes.

· After a pre-purge of about ten (10) seconds the electronic
ignition device will be energized.

· After a slight delay the gas valve will open.

· The burners will ignite and the flame sensor will detect
the presence of flame. The ignition device will
deenergize. If the sensor does not detect the burner
flame, the gas valve will close and the ignition cycle
will be repeated for a total of three attempts. If, after
the third attempt, the presence of flame is not detected,
the furnace will go into a lockout condition for one (1)
hour. It will then repeat the ignition cycle. This one (1)
hour lockout and retry will occur indefinitely.

· Thirty (30) seconds after the main valve is energized
the circulating air blower will be activated.

· The furnace will remain in operation until the demand
for heat is satisfied.

· Once the demand is satisfied the venter will shut off,
and the circulating air blower will shut off after the field
selectable time off is attained.

· The furnace will remain dormant until the next demand
for heat.

There is a limit switch that is likely opening. Be sure your filters are clean and all registers and grilles are open and not obstructed. If the problem continues, you may have a dirty indoor coil or undersized duct work. Some models have a window where you can observe the LED lights. If when the furnace shuts down you see any blinking, you need to count the flashes and we can figure out what the problem is from there.

If the unit fires but the main blower fan doesn't come on the the unit is shutting off on a temp high limit switch and it sounds like the problem is the main blower fan motor or control

Flip the lower panel over and look for a wiring diagram. On that diagram there will be a diagnostic chart. Let me know what 5 blinks represent. Then I want you to read on.

On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

limit switch is on the heat exchanger black plastic about 1and1/2 in. by 2 in. with 2 wires going to it , it has 2 screws holding it in take it out and check it with an ohm meter and you should not have a reading if it is good

You have a limit switch located in the furnace (a little bigger than a dime and located near the center of the front panel, sometimes the switch is mounted to a small fiberglass board) . There will be two wires going to it. They sometimes trip so often that they fail to automatically reset(due to dirty filters or undersized ductwork). You can SHUT OFF POWER TO THE FURNACE, then remove the two screws holding the switch in the furnace, pull the switch out of the furnace and tap on it and wipe it off.
The switch may work a few more times but should be replaced with the same size and type. You will see some rating printed on the switch somewhere. Write down all the numbers and get a replacemet from a furnace/AC supply house. If you have a meter and know how to use it you could check the switch for continuity.

Sometimes just a tap on the switch will get it going. There will also be a couple of rollout switches that could be tripped. They have little buttons on them to reset them. If they trip it may have other issues to be solved.
Hope this helps!
Matt

Your main limit switch is activated. Maybe for good reason. This switch keeps the furnace from overheating. I have the same problem right now and I think my a/c coil may be blocked, limiting air flow through the heat exchanger and the furnace. Does your gas and main circulating fan come on for a few minutes before the switch activates and the light begins blinking?

Check your furnace filter and any return air ducts in your home - see if they are blocked or restricted.