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Brian HVAC Guy
Brian HVAC Guy, HVAC Technician
Category: HVAC
Satisfied Customers: 916
Experience:  I have been an HVAC/R mechanic/technician for 30+ years.
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Mark I have a four flashes Open Limit switch problem with

Customer Question

Hi Mark
I have a four flashes Open Limit switch problem with my Concord Gas Furnace but not able to find the reset switch and unable to find the manual of the schematic for Model GC90UB075D16C.
Blower is on but no ignition is not on. No heat. 4 flashing light.
Thanks for your help.
Submitted: 2 years ago.
Category: HVAC
Expert:  Brian HVAC Guy replied 2 years ago.
Hi I'm Brian and I'm here to help. The four flash code does indeed point to a high limit switch open condition. Your limit switch should be upstream of the burners on the bulkhead separating the heat exchanger. If you follow the white wire on pin one of the 12 pin plastic plug on the control board you will find your limit switch. This is an auto reset limit control. You can try removing the power to the furnace for a few minutes but I doubt that will work. It's possible that limit switch has failed and needs to be replaced. If you can't determine which is pin 1, it is the pin on the lower right as you look at the board with the lettering right side up. The most common cause for a high limit switch trip is a dirty air filter or plugged evaporator coil. If the limit switch was cycling a lot due to those issues it's possible it stuck open. You can TEMPORARILY jump out the limit switch when you find it to confirm it's bad but NEVER EVER leave a limit switch jumped out to run the furnace!!! Click Here for an example of what a limit switch looks like. If I can answer further questions about this please reply and ask. Otherwise please remember to rate me positively before you leave tonight. Thanks, -Brian
Expert:  Brian HVAC Guy replied 2 years ago.
Click Here for another example of a high limit switch that is a different type. I'm having a hard time locating the exact switch that your furnace uses. If you find the switch and can give me the numbers off of it I may be able to help better. Thanks, -Brian
Customer: replied 2 years ago.
Hi Brian,
How do you temporary jump out the limit switch to check to see if that is a problem and see if the furnace will fire up.
the filter was dirty.Thanks.
Expert:  Brian HVAC Guy replied 2 years ago.
When you find the limit switch, it will have wires connecting to its two terminals. You just jumper those wires together. This is a low voltage 24 volt circuit where they connect. There may be one of the terminals with two wires on it while the other terminal has only one wire on it. If that's the case you have to make sure all three wires connect to each other. If you have a jumper wire with alligator clips you could use that while they are still connected to the limit, you can can also use a carefully bent piece of a paperclip to jump them (with either the wires still connected OR removed). If the female spade connectors are not insulated they will slide onto each other so you could remove them and connect them that way. I must repeat myself and caution you, DO NOT leave them jumped to get heat. It is dangerous to leave the system without that protection! Thanks, -Brian
Customer: replied 2 years ago.
Expert:  Brian HVAC Guy replied 2 years ago.
You're very welcome! Thanks for using Just Answer and allowing me to help you tonight.
Expert:  Brian HVAC Guy replied 2 years ago.
For your convenience Click Here for a link to the PDF copy of the manual for your furnace. :)
Expert:  Brian HVAC Guy replied 2 years ago.
Hello again. I'm just following up on our discussion last night to see how things turned out. Where you able to determine that is was a high limit switch issue? Is there anything else I can answer for you? If not, please remember to rate our session today. Thanks much, -Brian
Customer: replied 2 years ago.
Jumping the high limit switch did not start the flame.
There was 27 volts between the two wires. I go ahead and order the new switch anyway. The red reset button seems loose hanging down when it was in upside down and not staying up when pressed
Expert:  Brian HVAC Guy replied 2 years ago.
If there was 27 volts between the two wires while they were connected to the limit switch, that is indeed your problem and replacing it should solve your issue. Let me know if I can help further. You can continue to post on this same question for follow ups when you get your limit switch even after rating our session. I will get an email notice of your post and reply back as soon as possible. Thanks for allowing me to help and thanks for using Just Answer! Have a great day!
Customer: replied 2 years ago.
The 27 volts was when it was not connected to the switch, just the voltmeter.
Expert:  Brian HVAC Guy replied 2 years ago.
That is normal you would have 27 volts between the wires when not connected to the switch. If you reconnect them and no longer have 27 volts, you have zero volts, between the two wires connected to the two terminals on the switch, then that switch is closed and that is not the problem. This is how any of your safety switches would be checked when the unit is calling for heat. Let me know what you find and we might have to look elsewhere for a problem.