Hello,

I have a Goodman Furnace (model # GMP100-3). The furnace will not stay running. Here is the sequence of events for two different scenarios that are occurring.

Scenario 1
1. Thermostat Calls for Heat
2. Inducer starts
3. Pressure switch closes
4. Heating element glows
5. Burners Ignite (flames nice and blue, everything looks good)
6. Blower Comes on, House Starts warming up, but stops well before temperature is reached. There is no set amount of time for this (sometimes 10 seconds, sometimes 5 minutes)
7. Burners extinguish prematurely and blower continues to run.

Scenario 2
1. Thermostat Calls for Heat
2. Inducer starts
3. Pressure switch closes
4. Heating element glows
5. Burners Ignite and immediately go out. Just a flash of flame is seen with no rollout

Other info - For scenario 1, the diagnostic LED flashes three times which means "pressure switch failure to close" and to check venter, pressure switch, vent blockage. I got a new pressure switch and changed it out (no improvement). I pulled off the flue pipe and checked for blockage and it is clear. I checked the hose running from the inducer to the pressure switch, and it is in good condition. I have also cleaned the flame sensor.

I guess here are the questions - Is it possible for blockage further up the chimney to cause this? Could it be a bad inducer not keeping the new pressure switch closed? I also checked the opening in the inducer. I put in a small drill bit and it goes in freely for the first inch, then hits something solid, is this normal? Should I force the drill bit in further?

Right now, given the two different scenarios, could this be a faulty gas valve as the burners just extinguish at different times? What else should I check

Apologies for a lengthy post, but I wanted to try to give all the info.
Thanks in advance for your help,
Mike

I would check the flame sensor and clean it with some steel wool or sandpaper.

Thanks for the response,

I have cleaned the flame sensor. I thought that was it when the furnace was acting up as in scenario 1, but it wasn't...
Scenario 2 is different because the flames aren't even on long enough for the flame sensor to detect. They go off immediately as they ignite. No one to two second delay, they ignite and go out all at the same time. Also, I have noticed that the furnace blower will not come on when I switch the fan to "on" from "auto" on the thermostat, what does this mean? Do I need a new thermostat? I have tried many times and the fan will not come on when I switch the fan button to on from the thermostat.

I still also need to know about the inducer; should I force the drill bit in as far as it will go, or just check the opening?

Please help - 4-6" of snow coming here.

Thanks,
Mike

Checking the thermostat is easy, just short the red and white wire together either at the thermostat or furnace. The furnace should come on and stay on, or a least until it overheats.

That could be your problem. Somewhere you have a sensor that shuts the gas down if the furnace gets too hot. It could be bad. The furnace could be overheating because of dirty filters, blocked vents, failing blower motor, loose squirrel cage, etc.

The gas valve could be flaky. If you have power to its terminals, and the gas is off, the valve is bad.

Not sure about the drill bit. If you suck on the hose and you here the switch click, it is OK.

Labman,

Thank you for responding. I quoted you a lot in this post, so I put my answers in bold.

"Checking the thermostat is easy, just short the red and white wire together either at the thermostat or furnace. The furnace should come on and stay on, or a least until it overheats." I will try this. Every time the thermostat calls for heat, the furnace tries to come on, so the thermostat is doing its job I guess. I just think it is weird that I can't get the fan to come on by switching the fan to on from auto on the thermostat. Will the red and white jump tell me anything about the fan problem?

"Somewhere you have a sensor that shuts the gas down if the furnace gets too hot. It could be bad." Will look for the sensor and see if that is the problem.
"The furnace could be overheating because of dirty filters, blocked vents, failing blower motor, loose squirrel cage, etc." There is also a third failure scenario now. Sometimes the burners run for a while and go out right as blower is coming on. Do you think that the blowers are not coming on when they are supposed to, thus flames burn to long, thus overheating furnace, tripping limit, and then blower comes on through a call from the limit trip to cool down the furnace?
Filters are clean, vents are clear and open. Will check blower motor. What is squirrel cage?

Also keep in mind that many times the flames go out on ignition, so it may not be overheating.

"The gas valve could be flaky. If you have power to its terminals, and the gas is off, the valve is bad." I will check this - what equipment should I buy, just voltmeter? ohms?

"Not sure about the drill bit. If you suck on the hose and you here the switch click, it is OK." I was using the drill bit for the opening in the inducer, not the pressure switch. The pressure switch clicks when I suck on it, and it is brand new. I just read somewhere that sometimes the opening in the inducer motor gets clogged preventing the inducer from keeping enough suction on the pressure switch, and you need to clear it out. That's why I wanted to know about the drill bit and if I should force it all the way in the draft motor since I feel it hitting something.

Thank you,
Mike

Check your electrical connections especially the grounds going from the house panel all the way to the switch on the furnace and from there to the furnace itself. Then make sure all the grounds are clean and tight in the furnace from the manufacturer.

After reading all of this and the many different ways the furnace has acted I do not believe it is a secondary component. It is a ground or the circuit board.
Take a really good look at all the grounds and if you feel like it loosen them a bit then retighten to a good and tight ground. Any area you see any white powder make sure to clean that area good and then reground.

This furnace has presented to many different problems.

Thanks for the response,

I did not find any areas with white powder, and the connections seemed tight.

I have recently discovered one more thing that may shed some light on the whole situation. I was in the basement observing the furnace. I watched it run for a while, then it shut down. About five minutes later, a faint buzz came from inside the furnace (lasted 5 seconds) then the system came on and ran great - got the house up to temperature. This is the first time I have heard this buzz, it was barely audible. Does this tell you anything? I really feel that this is connected somehow, but I don't know what the buzz could have been.

Thanks,
Mike

Sometimes the spark ignition will buzz as it tries to lite the pilot.

The buzz happened while the furnace was shut off after malfunctioning on previous run. Then after the buzz, normal sequence started: Inducer, pressure switch, gas, HSI, Blower. Typically when the furnace malfunctions it locks itself out for an hour. This time it was only out for five minutes, made the buzz and then started up and worked great.

Actually your problem is beyond me since I am not there and all that is going on here is guess work. Your problem is intermittent in nature and very difficult to diagnose. Sorry I could not be of help.

Did you loosen and retighten the flame senser,for a good ground?

First jump out rw at the borad in the furnace, that's a call for heat.if this works replace t stat... high limit jump it out... check manifold gas pressure 3.5 " don't clean sensor change it out its cheap.. /. Last goodman is a contractors special designed to last notttt call goodman tech support hehe if there is any.. seriously try whaT I SAID.. T STA CAN Cause YA PROBLEMCHECKS ALL FEMALE CONNECTORS FOR TIGHTNESS SPECIALLY GROUNDS.. OR HIRE ME ILL FIX IT FAST... BOSTON

Replace the HSI. They are reasonable priced. Just went through this scenario at my neighbors house. There furnace was also a Goodmen and the HSI glowed but would not lite off it would just shut down. I told him to pick one up on Friday and put it on Saturday. Still working. Caution do not touch ignitor end the oil on your skin will damage it.

I see last post was Dec-2007,

But one area to check is the main multiplug for corrosion.

Multiplug location, lower access panel, right side on the horizontal sheet metal shelf that separates the upper an lower compartments.

.

OP here on a 5 year old post and I wanted to follow up and say the furnace is still going strong!

Lots of good advice from this board, but I have to give hvac1000 a long overdue special shout out. My problem wound up being the circuit board. I replaced the HSI, pressure switch, flame sensor and circuit board. Did it all myself for less than $200 and the furnace has done great ever since. I think the many different scenarios should have pointed me to the board earlier, but other parts were cheap and easy to replace.

Thanks again everyone! I am getting ready to post about grandparents furnace so hopefully I have good luck again.

Mike