Goodman GMS80704bn gas furnace - light blinks 4 times

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Old 12-22-15, 04:30 AM
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Goodman GMS80704bn gas furnace - light blinks 4 times

Hi,

3 days before Christmas and the furnace goes...ugh!

After reading through the posts, it appears the limit switch is bad on my Goodman furnace (light blinks 4 times). I see the limit switch B13701-88 - which appears to be the primary one for this unit. It doesn't look like the ones I've seen which have a plate and then a long piece with the switch at the end.

How can I tell for sure that's it the limit switch? No, I don't have a meter but will go get one today if I need to. If I need a meter, what exactly is it called?

thanks
 
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Old 12-22-15, 06:46 AM
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Limit switches rarely go bad. Might be a airflow problem. Make sure you have a clean filter in it.
 
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Old 12-22-15, 06:53 AM
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A clean filter in the furnace? or around the furnace? not the filters that are for the intake that are in the ceilings in the house, right?
 
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Old 12-22-15, 11:23 AM
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I can't find a filter on my furnace. There's a sticker that says "this furnace does not have provisions for installing circulating air filters inside the casing. All filtering must be done external to the casing. When filters are installed in the same pressure zone as the furnace, any return air openings must be sealed airtight". So, no filter.

Could it be the switch?

Thanks
 
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Old 12-22-15, 11:45 AM
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A multimeter measures AC and DC voltage, AC and DC current and resistance and may have other features. They can be purchased for $20 or so and have a lot of uses around a home.

It's very unlikely that the limit switch is bad. It's probably opening because the furnace is overheating ----usually due plugged up air flow through the furnace.

And the leading cause of THAT is a plugged filter. If there is no filter in the furnace, there is usual;ly one on the return air system, often at the return air grill. You need to check that out and look for a filter which may be plugged with dust and dirt.
 
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Old 12-22-15, 12:18 PM
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Yes check the ones in the ceiling.
 
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Old 12-23-15, 07:27 AM
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I change those filters regularly. The last filters I had said 90 days and I did change them when the furnace first stopped working. I moved into the house in May and have changed them 2x so I was probably a little over a month overdue. Now that the filters are changed, the furnace is still not giving out hot air. It does come on, the blower sends out cool air. There is also a smartzone-2 controller but not on the furnace - if that matters.

I will definitely get a multimeter today.

Any other suggestions would be greatly appreciated.
 
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Old 12-26-15, 01:16 PM
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Yep, that part number is the automatic reset limit switch:
http://www.amazon.com/B13701-88-Good.../dp/B00FFB94L0

What you need to do is to use a multimeter to check to see if the limit switch is in fact opening and shutting off the burners.


First, carefully observe the sequence of events which occurs when you turn up the thermostat, and post that sequence here, in order and in detail.

Then use your multimeter to measure the voltages applied to the limit switch and see if the limit switch is, in fact, what is shutting off the burners.

When the thermostat first calls for heat, you should find 24 VAC to both sides of the limit switch --- which proves that the limit switch is closed, as it should be.

When the burners shut off and the thermostat is still calling for heat, do you find 24 VAC to only one side of the limit switch, or to both sides?

If you have voltage to only one side the limit switch has opened and the furnace is overheating. When the limit switch open, it shuts the burners off but the fan circulating air around the house typically stays on to cool off the furnace.
 
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Old 12-29-15, 05:40 AM
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Smile

Ok got my multimeter and will check tonight. I did turn the heat on last night and it fired right away. This morning, it was not. I have a smartzone-2 control and I can tell no heat when the temp on that is below 70 degrees so when I looked this morning and it was on 66, I knew the furnace was not blowing hot air. The fan was working though.

I don't suppose it has anything to do with having the heat set on the 1st floor but ac on the 2nd floor, does it? I have a dual system so I'm guessing not but thought I'd ask anyway.

Thank goodness for 70-80 degree weather in December!
 
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Old 12-29-15, 05:46 AM
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Also, there are 2 switches on my furnace - one on the blower and one by the ignition.
 
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