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I have a Goodman 80% furnace in my barn dated 1999 it runs for about 5-10 minutes then flame kicks off for about 2 minutes and re-lights. It does not go into lockout mode (keeps cycling this way). When t-stat reaches called for temp furnace will shut down til called for again.

I have replaced the limit switch already. Flame sensor would not be the problem because it heats for 5-10 minutes at a time, however I did clean it with scotchbrite pad to rule out that possibility. I am using the bottom of furnace for cold air and no filter. Furnace is hanging 42"above floor.

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    Exhaust blower sensor may be starting to go. Some rely on a pressure differential across a rubbery membrane. Rubber can get stiff over 20 years, so contacts may no longer reliably close. That'll turn off gas. – Wayfaring Stranger Jun 20 '18 at 22:40
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I had a similar problem with my furnace, which was a newer more efficient furnace so not sure it applies in your situation. I did a lot of trouble shooting and it ended up being the condensation in the mouth of the tube that goes to the pressure switch.

In my situation I could make it cycle by momentarily (approx 1-5 seconds) pinching the pressure switch tube, and the furnace would restart but not lockout.

After I worked out which pressure switch was having the problem, one option was to drill out the plastic a little bigger so that the condensation wouldn’t effect it as much. However I didn’t want to do anything permanent, so I experimented with putting a cut down bit of plastic straw into where the pressure switch tubing came out of the furnace, to make like a cap inside so any drips of condensation couldn’t block the entrance to the pressure switch tube momentarily. It has worked perfectly for 2 years since adding the piece of plastic straw.

It’s a little hard to explain so feel free to ask questions if you think you might have a similar situation.

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The control board may have an led that flashes a problem indicator. My Goodman 92% lit up and shutdown. The led flashed "not normal". Replacing board fixed the problem. I used an after market equivalent that I was able to find by cross-referencing p/n.

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It's possible that your flame sensor needs to be cleaned or is on the outs. Some plagiarism from the linked article there..

..signs that your flame sensor is bad include:

  • Gas burners light but then go out after 3-4 seconds
  • Visible soot is covering the tip of the flame sensor
  • The white porcelain on your flame sensor is broken or cracked

Where is my furnace’s flame sensor and how does it work?

Your furnace’s flame sensor is a short, thin metallic rod that’s either straight or slightly bent and sits inside your gas burner assembly. Its sole job is to sense whether the gas burners light to create a fire or not.

If you're feeling adventurous you can try to clean the flame sensor yourself. I recently did this for the first time and my furnace wasn't too challenging. I recommend Googling your furnace's make/model to look for images of the flame sensor, or videos of people servicing your existing model. In my case, I have a "Tempstar" furnace, but its internals are all "Honeywell"

A few IMPORTANT notes, though:

1) Make sure you throw the furnace breaker before servicing it. Some of the wiring might be exposed behind the cowlings and it has the potential to zap you.

2) Your igniter element and flame sensor element (particularly the igniter from what I've read) are sensitive to the oils in your skin, and you can shorten their life inadvertently by touching them. Consider wearing some latex gloves if you have a pair.

3) You'll want to clean the flame sensor element with something relativly soft (i.e. NOT sandpaper). In one video I watched, the person used emory cloth, which is probably perfect. I used a new, everyday, green, kitchen scouring pad. Go slowly and gently.

Honorable mention: Depending on your furnace, you might want to pick up a spare igniter and flame sensor to keep on hand if these are not too cost prohibitive, as it can be hard to find them when you need them on a Friday night or a Saturday.

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It's entirely possible that nothing is wrong with your furnace and this is normal. It might be heating up and then kicking off when the high limit is reached and turns back on after it circulates air and cools down a bit.

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