Open limit switch on Goodman furnace
I have a goodman GMT045-3B furnace that I've had lots of issues with. The latest is a problem when I turn the heat on. The flame ignites, but the blower will not cut on. The flame then cuts off. The control board light is blinking 4 times when this happens. The chart on the panel says this means there is an Open Limit Switch and to check the main limit open or aux limit open. I don't know much about this stuff. Is the limit switch integrated with the control board where I'll have to replace the control board? How do I go about fixing this? Thanks in advance for your replies. First, just to eliminate a common problem, turn the furnace off, remove the flame sensor rod and clean it by wiping it gently with very fine steel wool or emery paper. Even Scotch Brite if you have it. Reinstall and turn on and test furnace. The limit switch is separate from, but feeds, the control board. I'm not sure where the flame sensor rod is. I have found the 2 limit switches and see that they would be easy to replace, but I'm not sure if it is that or the board. Here is some additional info. If I put the fan from auto to on, the fan comes on. However, when I then turn the thermostat so that the heat comes on, the blower shuts off. I went up to the furnace after this cycled, and the small blower that purges the air before the gas comes on was very hot to the touch. It sits right above the flame, so it's understandable why it was hot. So I'm assuming that limit switch going open at 200 degrees is probably correct, and is working as it should. Again, the flame burns for a while before it shuts off...and when it shuts off, the board starts to blink indicating a limit switch. So am I right thinking that the limit switch is doing its job, and the blinking light is just indicating that it detected the switch opening? Since the fan goes off when the heat comes on, would that more indicate a problem with the control board? Are you saying the fan does not come on until after the flame shuts down when you have the thermostat set to 'auto'? If this is the case, it would seem you either have a problem with the fan motor (not likely) or the circuit board. Here is a little more information: 1. I put the fan in on position. It comes on with no problem. 2. After a few minutes, I put the heat so that it will come on. 3. The burner ignites and the fan is still blowing. 4. After about 1-2 mins, the fan stops, but the furnace is still going (flame is still going). 5. When this occurred, I went and checked the light on the board. It was solid indicating no issues. 6. After a few more minutes, the limit switch will trip shutting down the flame, and the board blinks indicating the limit switch. So, by having the fan on, wouldn't that bypass the lower limit switch (the one that comes on at 120)? And since the blower stops while the flame is still on (and board is showing normal), wouldn't that eliminate the upper limit switch? So what else, besides control board, could it be? I'm assuming it is the control board. Originally Posted by Grady Are you saying the fan does not come on until after the flame shuts down when you have the thermostat set to 'auto'? If this is the case, it would seem you either have a problem with the fan motor (not likely) or the circuit board. The fan won't come on at all in auto mode (except for the small fan that purges the air). But the blower doesn't. This motor was just replaced 2 months ago, and works fine with the AC...so I'm certain it isn't the blower motor. It also comes on in on mode. It just shuts off when the heat comes on. So it has to be the control board, right? It certainly sounds like the board. If you have a voltmeter you can check to see if you ever get 120 volts out of the heating speed fan terminal. If you don't, it's almost certainly the board. If you do, it sounds like the motor. Was the motor's capacitor replaced at the same time? It should have been. Originally Posted by Grady It certainly sounds like the board. If you have a voltmeter you can check to see if you ever get 120 volts out of the heating speed fan terminal. If you don't, it's almost certainly the board. If you do, it sounds like the motor. Was the motor's capacitor replaced at the same time? It should have been. Yes, there was a new capacitor put in 2 months ago. Thanks for the response...it is still under warranty, so I'll get the board replaced. So it was the board, but it was an error on my HVAC guys part when they replaced the fan motor this summer. He connected the hi speed terminal to the cool terminal on the board, but didn't connect either the medium or low speed to the heat terminal. So when I switched to heat, it couldn't cut the fan on. I connected the medium speed connector to the heat terminal and it works fine now. Thank you for the responses. Just so I'm clear on the solution, did you have to replace the board or just hook up the wire? I wish I could say I've never forgotten to connect a wire. Originally Posted by Grady Just so I'm clear on the solution, did you have to replace the board or just hook up the wire? I wish I could say I've never forgotten to connect a wire. I connected the medium speed (blue wire) from the blower to the control board. He didn't connect either of the remaining wires to heat. He only connected hi speed to cool. So when the heat came on, the board couldn't turn on the blower, which caused the limit switch to heat up quickly. I did not have to replace the board, only connect the wire. Originally Posted by ncsu95 I connected the medium speed (blue wire) from the blower to the control board. He didn't connect either of the remaining wires to heat. He only connected hi speed to cool. So when the heat came on, the board couldn't turn on the blower, which caused the limit switch to heat up quickly. I did not have to replace the board, only connect the wire. And as soon as he called me back this morning (after I described the problem), he realized what he had probably done and told me what to look for. Sure enough, wire wasn't connected. Originally Posted by goldstar First, just to eliminate a common problem, turn the furnace off, remove the flame sensor rod and clean it by wiping it gently with very fine steel wool or emery paper. Even Scotch Brite if you have it. Reinstall and turn on and test furnace. I had same symptoms on Goodman 125-5: cycled thru ignition but each time flame cuts out after about 5-10 sec (before blower gets started). Goldstar's solution worked for me -- thanks! Hello all,first off last week my furnace led was flashing 3 times calling for pressure switch open,called a pro and they came out,found out heat exchanger was bad,they replaced.now 3 days later(and never had this problem till they replaced heat exchanger) the exchanger comes on, lights burners, fan comes on, furnace runs,then after a short while the high limit is tripped and light blinks 4 times but after cooling fan runs for a while light goes solid and burners come back on.heating house ok but I know this is not right.I called the pro that came out and he said this is a seperate issue and not related.I think it is bull s##t.There is no air flow restriction to my knowledge.I cheched everything.Please help.could I have a sensitive limit switch not familiar with a brand new heat exchanger.furnace is 4 yrs old. If they replaced the primary heat exchanger, maybe the old limit switch is not compatable, and maybe is too close to the new exchanger, giving a false reading of too high. I'd check with the guys who installed it if there is a possiblity of that. On some control boards, not sure about yours, have dip switches to adjust when the fan comes on goes off.
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