[cplnorton]

Ok, I'm not a expert but I've worked on my own furnace several times, so bear with me.

Goodman furnace GMH950704cxac, that is probably about 6 or 7 years old now does not seem to be getting gas.

Everything starts up normal, I can see the ignitor turn red hot, but there is no fire at all. Sequence repeats three times and then the board flashed one light which means lockout.

So I suspect it is a gas problem. I have a gas log set on the the same gas line and it works fine so I do not think there is a gas supply problem.

When I disconnect the plug to the gas value, there are three wires labeled M, C, IH. When I connect the C and M, I get nothing at any time. When I connect the C and IH, I get 24V for a brief moment at the very end of the ingnitor sequence.

That is one thing that seems very strange to me, I do not get the 24V until the ignitor is basically almost not red anymore. In fact the ignitor is almost off before it happens.

Also shouldn't I get the 24V at the C and M, as the M is the low. And the IH is the HI in the 2 stage?

I have a feeling I should be getting 24V at the low and that would mean a wire or board problem.

If I am wrong and I should be getting 24V at the Hi, then it's either trying to late or its a bad valve.

Any help would be greatly appreciated. My cranky wife would love to have heat again. lol

 

[Bitters]

Most Goodman's start on hi fire and then go to low. Are you sure it's a 2 stage. Manual says it's single stage.

 

[cplnorton]

Ok I just went down and made sure. This time I'm getting 22 to 24V at the high side to the gas valve over the 3 cycles. I can hear the relay on the board click, then the voltage shows up on the meter. I cannot hear any valves opening in the gas valve. When the voltage stops, I do hear a small sound from the gas valve. But I do not hear anything when the voltage starts.

So I'm guessing bad gas valve then?

Thanks for your help!

 

[roughneck]

What's outlet pressure on the gas valve during ignition trial? What's inlet pressure?

 

[Bitters]

Try testing C to PM.I wouldn't condemn the gas valve yet. Ae you getting trouble code?

 

[cplnorton]

On testing the outlet and inlet pressure of the gas, you have to have a special tool don't you? If it's what I think it is, it requires a tool I don't have. I wish I did though.

No trouble codes, only the single flash of the lockout when it goes through it's 3 cylces and doesn't lite.

Bitters, when you say test C to PM, do you mean doing a ohm test on the vale solenoid itself between the C and M on the valve?

Thanks for your help guys. I'm more a car guy than I am this kind of stuff, so I greatly appreciate everyone helps in trying to fix this.

 

[roughneck]

You need a manometer for reading gas pressure.

 

[Bitters]

No, test for voltage from C and PM.

 

[cplnorton]

Bitters I think it is a 2 stage. I double checked the number and it is right that I listed above

 

[cplnorton]

Bitters sorry my internet is so slow, I didn't see you posted a new response. Now I've seen some gas valves that have PM marked where mine is marked M. So by PM, you mean test the M and C for votage on the ignition cycle, is that correct?

When I test the M and C on mine during the cycle I get nothing on all three cycles.

This is how my valve is marked.

thanks for your help!

 

[roughneck]

You'd test C to the other two terminals, looking for 24V AC.

 

[cplnorton]

When I connect C and HI together with the test leads, I have the 24V at the end of the cycle.

When I connect C and M together, I have nothing at anytime.


That is what you guys mean right?

 

[roughneck]

Yes, the valve is trying to open then. Next step is testing gas pressure.

 

[beenthere]

Yes, the valve is trying to open then. Next step is testing gas pressure.

Low fire has to be energized for the gas valve to open. No voltage to M, means no ignition, because the internal redundant valve won't open.

 

[cplnorton]

beenthere, so I need to have 24V at the M as well, which I don't. So then I'm looking at a wiring or control board issue?

 

[beenthere]

beenthere, so I need to have 24V at the M as well, which I don't. So then I'm looking at a wiring or control board issue?

Yep. Could be a loose wire in the mole on the board, or the board.

 

[cplnorton]

Ok an update because I know these posts can help someone else later on in a google search for the same problem.

You do have to have power at the M and C, which is your low. So you need 24V at both the high and low on this furnace to get the valve to open for gas.

I jumped the Hi to low on startup and it fired right up. So I went back and checked every wire connection, and unplugged and plugged them all back in. It must have been a wire that didn't have a good connection because it's working again.

Right now when I test it, I have 24V at M &C. And I have 24V at C and HI at startup. After a little bit, the HI side cuts out, and the low side is the one that continues to run. But for starting both have power. So it's exactly what it should be.

It's at least working now and it has been for the past 12hrs or so. If it cuts out again I'm guessing it's the board. I might even be proactive and buy a board and put just put it back for a rainy day. I've heard they are very prone to going out on these goodmans, so it might not be a bad part to lay back.

But for right now it's all working, so thanks everyone for their help.