[eggs]

To start, I am a professional of 40 years in the trade, so you came to the right place - this is not uncommon with Goodman furnaces. So let's start from the very beginning on how the unit works. Then how to test each step.

1) Call for heat (R makes to W) - W now has 24 vac on it to ground (C) - need to confirm it is not being disrupted (I realize odds are not, but it has happened)
2) Before the venter motor comes on, the air switch must be open - failure to be open gives an error code.
3) Venter motor starts - the air switch will close - proving the venter motor is running (vacuum of 0.7" WC (0130F00505) (0131M00002PSP)
4) Once the air switch closes there is a short delay and the hot surface ignitor gets hot (B1401018S)
5) Aprox 3 seconds later the main gas valve opens, and we should have ignition. (B1282628S)
6) Control board is looking for a DC Micro amp feed back from the flame rod which is sent out a small AC power - rectifying the power to DC (0130F00010)
7) If after a few seconds - if flame proof is not proven, gas valve closes.
8) If at any time through or after this we loose our -0.70" WC venter motor proof, the gas valve will close.
9) If at any time through or after this we have flame roll out - the gas valve closes.
10) If at any time through or after this a high limit opens, the gas valve closes.
11) If at any time we loose our flame rod proof of flame, the gas valve closes.

Now we understand that there is more than one thing that can cause your issue.

Lets break it down to the most common issues Goodman has with the 80% eff furnaces.

1) Bad heat exchanger - I have seen this on units from 1 year to 5 years old
2) Bad air switch - Very common on all furnaces
3) Cracked or broken hoses from venter motor to air switch
4) Cracked fitting on venter motor to hose connection going to air switch
5) Plugged exhaust pipe to chimney
6) Plugged chimney
7) Plugged venter motor fan blades
8) Venter motor leaking air around it, not pulling air out of the heat exchanger
9) Bad high limit switch
10) Bad flame roll out Switch
11) Improper voltage to unit
12) Poor grounding
13) Bad control board

In this order is the list of most common, note the control board and grounding are last.

The GMT series should be a 80% efficient unit with a standard tin pipe going to a chimney, NOT directly vented out a wall.
I've seen this mistake a few times and it almost killed pets and people, so I assume it is exhausting up a chimney and the vent pipe is clear, chimney is clear. (Seen Birds in the pipes)

Before the first test, we need to explain a few definitions - I can only assume terminology we use is uncommon to most, so we will start there.
Inches of Water Column ("WC) - is how we measure very small amounts of pressure. 27.72" WC = 1 PSI. We know it takes .433 PSI to push water up a tube 1' or 12", when converting it, you would come up with the 27.72" = 1 PSI. One would not measure distances from New York to Arizona in inches or feet, we use miles. One would not measure the distance from Earth to the next near by solar system in miles, we use light years. In our case we are using a very small pressure, 0.70" WC or 0.0253 PSI. Gas pressure to the furnace manifold (burner) is also measure the same way. If you are using Natural Gas, it would be 3.5" WC on high fire and 1.75" if you had 2 stages on low fire. If you had LP it would be 11" WC on high fire.

Now we understand pressures. The air switch is measuring the vacuum the venter motor is creating. In your case we should be able to pull around 1.2" WC, and if we do not, we have a problem.
This is the most common issues of all furnaces, but now to understand what the problem is.
I'm sure you do not own a manometer that can read these small pressures, so let's attack it in a different way. First thing we do is very carefully remove the hose from the venter motor going to the air switch. Next, while still on the air switch careful suck with your mouth slowly on the tube and you should hear the clicking of the air switch. Your mouth is capable of pushing and sucking over 2 PSI (55" WC), so we do not want to damage it, as it will not take much for this to happen. With the hose off, start the furnace up. once the venter motor starts, start sucking a continuous vacuum on the hose, remember it will not take much. If the furnace starts up and stays running, you now have eliminated 9 - 13 of my list. If this is true, then we have 2 options, best option is to replace the switch and see if that resolves it. IF it does, I just saved you a several hundred of bucks. If it does not, we need to start going though the other possibilities.

Until we do the above, I will not go through the next step - this gets to be a long process - wish I could get paid for this as it does take a lot of my time, but it is what it is. Retirement leaves me with spare time to help others...

Amazon is one of my go to places to send people for parts, they have them on Prime for $21.

Amazon.com

Let us know how you do, hopefully this solves it, and hopefully you do not have a bad heat exchanger. I worked on one of these 2 months ago, had to tell the lady her heat exchanger failed even though it was 5 years old. Either a new furnace or replace the exchanger. Warranty only covers the cost of the parts, not labor and I can install a new unit in less time than replacing the heat exchanger which could fail again. After having this happen 2 times in 2 years, my opinion of Goodman has gone from a rating of 8/10 to a 3/10, would rather see a Carrier installed before a Goodman and that is a low blow. I sent the lady back to the installing contractor to handle, they should do something for her. I are my $21 air switch and 4 hours of messing around, did not have the heart to charge her.

Thanks

Eggs

 

[eggs]

SO, I still believe the issues are in the air switch tripping. Like I said above, you need to prove it.

Either after the venter motor starts, jumper across the air switch, or use your mouth to maintain a vacuum on the switch.

If you pick up a manometer, it must be able to read 0.01 through 9.00" WC - I see a few on Amazon for $42, no clue how good they are as my Dwyer 475-0 is around $300. The AAB SPM-100 I own works very well also and is around $100.

If the venter motor can not hold -1.2" WC, then you have a heat exchanger issue. What you may find is the unit is holding around -0.60" to - 0.85" WC and after ignition it just falls for a split second, thought to kill the unit. That is a sign of either leakage from the venter motor, failing venter motor, bad blade on venter motor or bad heat exchanger.

As for a replacement in the future, a good unit that is not real expensive is a Decane, an off brand of Lennox. Strongly suggest a 90%+ eff, but you will need to install a chimney liner for the water heater.
Lennox also makes a good unit, but then you are paying for the advertising.....
Trane is the best I have seen while Carrier is the very last. The reason why we rate Carrier so poor is they still have secondary heat exchanger issues on the high efficiency units and support along with parts are not only expensive, but can be hard to get. Lennox series brands are fair priced, parts are easy to get.

 

[eggs]

SO, Are you getting any error codes?

IF you waited for the venter motor to come on and jumper the switch closed and the same results =
Another safety is tripping or you are not getting 2 micro amps DC signal back from burner.

This would leave us with:
1) SO = Stack Over Temp Switch
2) PS2 = Pressure Switch Combustion Box
3) ROS = Roll Out Switch
4) ROS = Roll Out Switch
5) ROS = Roll Out Switch. (Total of 3 per Print)
6) PS1 = Pressure Switch Box

In the next test you will bypass most safety controls and see the results
Jumper the outlet of PS1 to th Gas valve power.
This will eliminate all but the first pressure switch.

See the PDF attached - Page 58

Otherwise you need a Micro amp meter that can read 0.1 to 9 Micro Amps
You should see 2 to 4 Micro amps in series with the flame rod.

 

[eggs]

Well, one of the safety controls are doing it, the interruption only has to be for a split second, then it can close again. Still could be a roll out switches. Bypassing all safety controls except the first one will eliminate all but the first one. If it runs with all but the first one bypassed, you limited down, but if it still happens then the first PS1 is the problem, then bypass it. If all are bypassed and it still happens, then the flame rod is the issue, either the control board or the rod, which only a Micro Amp meter that can read the 2 - 4 Micro Amps DC would be able to read. If it read 2 to 4 and still happens when all safety switches are bypassed, then it is a bad control board.

Sounds like a lot of work, but normally I have these issues solve in about 30-40 minutes as a professional with the correct instruments. Just take your time, and NEVER leave a safety switch bypassed for any length of time or while you are not infront of the unit with the power switch with in reach.

If the code keeps coming up with failed ignition, I would be leaning to flame rod issues, but that code would also come up if flame was not detected for a constant amount of time.

Amazon sells a cheap meter that may be a good investment for you. I am not a fan of these off brand made by who knows with labor that makes $1 a day, but as a home owner that does not require precision, they do the job. Search Amazon for0 "AstroAI DM600AR"f for $27

Amazon.com

 

[eggs]

OK, tests showed a few things. Waiting for the correct meter as you can not read micro ams with a milli volt scale or a milli amp scale. 1 micro amps = 1,000 milli amps.

You can do one of 2 things next, other than read the flame rod.
1) You can purchase a few Tattle-Tales. or
2) Jump out each thermal disk and test.

A Tattle-Tale is a one time use device - a resettable version is available but more expensive. I have used both and they do work well.
The one time shot versions sells for anywhere from $9.00 each to $57 for a pack of 3. Need to shop that around (#WGTT24), While the resettable version (#TC-007) $75 on Amazon.

Either one of the above, jumped across one of the thermal control switches, would identify which one is the issue.

Another method would be to jump out or by pass one thermal switch at a time, then start up and see if it runs. This can not be done quite so simply on an air switch, as the switch can only be bypassed once the venter motor starts up. Once running, it then can be bypassed. Only do one at a time to start as there may be multiple safety switches failing. If only jumping out 1 and no results, then you need to repeat by bypassing more than one at a time until you identify which ones are causing the issue. This still could be an air switch caused from heat exchanger issues. Only a manometer could test this. Vacuum on an air switch should pull about 25% - 50% more than the rating of the switch. All it would take is a fraction of a second vacuum loss to shut it down.

Also note: Just because a control is brand new does not mean it is 100% reliable, I do get bad switches from time to time.

 

[eggs]

f all the safety switches are by passed, the air switch bypassed after the fan starts up and you still have the same results, that leaves the flame Tod or control board.
If the signal is 2 - 4 Micro amps, then it is the control board. If replacing the control board and it still happens, then we are back to one of the switches some how.

 

[eggs]

1.9 is low enough to trip it. Take out the flame rod, use 220 sand paper to clean the rod, make sure it is in the flame good, not too far off to the side.

 

[eggs]

You can check gas pressure, but normally the pressures do not change unless you have a gas valve problem, not rare - as I have seen issues with them. To check gas pressure you will a good Manometer that can handle a range of 0.00 to 19.99 inches water column ("WC). Providing this is a single stage unit, the gas pressure on the manifold should hold a constant 3.5". Should not drop below 3.4" or go above 3.6" WC. If you have a 2 stage, low fire in most furnaces should be around 1.8" WC. You could pull the burners out and wash them in warm soapy water, make sure they are clean. Rarely do I see that an issue, but the environment exposed to the furnace will effect it.

Drop out on most furnaces is around 1.5 micro amps to 1.9 micro amps. I try to target some where in the area of 2.4 to 4 micro amps on units like this.

So the next step now that you know where you sit, wash the burners - be careful, easy to do - just loosen the union to the gas pipe, remove the cover to the burners if there is one - and remove 4 screws on the manifold and you should be able to pull the burners out. TAKE PICTURES as you do this as a reminder on where it all goes back together. I forget once in a while and end up scratching my head for a few monument in attempts to recall, work on so many units - no one can recall perfectly. Lining up the burners in re-assembly can be a challenge and new expressions and words can come out, so young ears may want to be else where. Help holding them in and get it all lined up may also be needed. After you do it about 1,000 times - it still can be a pain, but you learn a few tricks.

At a least resort - you can replace the board. Every thing you are going through is pretty much what we do, all in about 1 - 2 hours to test every thing, but this is a good learning curve for people wanting to learn this stuff. Too many people try to tell us this is too dangerous - to complicated, but when you have good guidance, this can be pretty simple, too many safe guards in the system to really cause an explosion. Always keep in mind your number on safety tool you own. Natural gas has an added oder and you can smell it easy, always go back a few times smelling around the unit so there are no leaks. Natural gas is lighter than air, unlike LP. LP will build up like a lake of water in a basement, then when it sees a flame, well - bad things happen, takes a lot more Natural gas for that to happen, just take care and always check the system after working on any gas pipe.

 

[eggs]

I'd Replace the board. 2.6 should be able to keep it running. Everything else seems to check out OK.

 

[eggs]

Great, glad you got it running..... Now you are a furnace expert!