Goodman GMH95 Gas Furnace Won't Start

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Old 12-02-15, 06:45 PM
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Goodman GMH95 Gas Furnace Won't Start

I have a Goodman GMH95 (95% efficiency) gas fired furnace that shuts down during the heating mode. The thermostat is working. The ICM performs the safety circuit checks. The induced draft blower is energized but shuts off after a couple of seconds and the ICM resets. It keeps repeating this sequence. Every once in a while the blower will complete the 15-second pre-purge, the igniter will start and the gas will light. But it shuts down and the ICM resets before the the circulator blower is energized.

I checked the error code and I'm getting 3 flashes followed by 6. I checked the drain trap and found it full. I emptied it. The pressure switch is fine. The flue and inlet seem fine. The draft inducer continues to run longer if I open the the thermostat contacts so I don't think it's the draft inducer. The fuse is fine. I suspect the rollout limits.

Any thoughts?
 
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Old 12-02-15, 06:48 PM
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Just bridged the rollout limit connections and no difference.
 
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Old 12-02-15, 07:34 PM
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Welcome to the forums.

Start from the beginning of the operation.
1) thermostat calls for heat
2) draft inducer blower starts
3) the pressure switch closes
4) the igniter starts to glow
5) 30 seconds later the burner lights and the igniter stops glowing
6) main blower starts

In your case the draft inducer is not staying running. That's a problem. You need to check and see what is losing power..... the draft inducer motor.... the control board.

The induced draft blower is energized but shuts off after a couple of seconds and the ICM resets.
That is a puzzling statement. How do you know the board is resetting ?
 
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Old 12-02-15, 09:40 PM
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Thanks for your help. When I said the board resets, I just meant it shuts off and restarts the whole sequence over.
The sequence goes through steps 1 & 2 of your post. How do you know when the pressure switch closes? Most of the time the igniter does not start to glow, but occasionally it will but the gas does not light.

I suspected it wasn't the inducer because it would run longer when I'd open the thermostat contacts.
 
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Old 12-02-15, 10:31 PM
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I take back what I said about the pressure switch. I was actually confusing it with the blower door interlock switch. I'm noticing a faint "click" from the pressure switch at the exact moment the heating sequence fails. I put a stethoscope against the pressure switch and it's definitely the source of the "click".
 
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Old 12-02-15, 10:42 PM
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After the call for heat.... the inducer MUST remain running. After it runs for several seconds the pressure closes, starts the prepurge and the start of the igniter to warm up.

So where is the chain failing ? Does the inducer remain running now ?
 
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Old 12-03-15, 01:42 PM
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The inducer, everything, including the l.e.d., shuts off the moment I hear the click from the pressure switch.
 

Last edited by bullfrogslayer; 12-03-15 at 01:45 PM. Reason: Incomplete
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Old 12-03-15, 05:25 PM
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You need to use an AC voltmeter to determine whether the pressure switch is closing and staying closed as long as the thermostat is calling for heat and the inducer motor is running.


It sounds like the pressure switch isn't staying closed reliably, but you need to check for 24 VAC to identify what is happening.
 
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Old 12-03-15, 05:32 PM
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This is a strange problem. Even if the pressure switch were intermittent it shouldn't shut down the inducer and it certainly shouldn't shut off the board. It almost sounds like a short to ground on the "switch out" line in the pressure switch.

You will definitely need a meter for this repair.

A thought just hit me.... do you have a humidifier ?
 
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Old 12-03-15, 06:01 PM
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I don't have a humidifier.
I just tried switching out the ICM and the same thing is happening. But now I'm not getting an error code at all. I guess the 3 and 6 flash error codes were old, from the repair history. I have a volt meter. What shall I do with it?
 
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Old 12-03-15, 06:27 PM
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Set the multimeter to read ac voltages in a range greater than 24 VAC.

Connect one test lead to the furnace sheet metal as a ground.

Use the other test lead to measure the AC voltages as needed.

In particular, check the voltages at the R and W terminals.

The R terminal should have 24 VAC when the power is turned on to the furnace.

The W terminal should be energized whenever the thermostat is calling for heat, and when there is 24 VAC to the W terminal the inducer motor should turn on and stay on until the thermostat is satisfied.

If the inducer motor isn;t staying on, measure the 120 VAC across the wires to the inducer motor. You should have 120 VAC to the inducer motor to turn it on and keep it on as long as the thermostat is calling for heat.
 
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Old 12-04-15, 02:21 PM
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Ok. So I was going to do what you suggested but before hand, I tested voltages on the pressure switches. The blower pressure switch had voltages of 1.3 and 25.8. The coil cover pressure switch read 0.0's. I also attempted to check the voltage on the flame sensor. As I did so, the circulator blower kicked in for a moment. Then I tried turning up the thermostat and the heating sequence was successful!!! The furnace is now operating normally.

What do you suppose happened? Could I have somehow unwittingly corrected a short?
 
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Old 12-04-15, 04:28 PM
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Could I have somehow unwittingly corrected a short?
Not a short.... a short will usually blow a fuse.
You may have come across an intermittent/corroded connection.

The coil cover pressure switch read 0.0's.
Not sure what that is.

The way to check a pressure switch is to connect your leads to the two terminals. When the furnace is not calling for heat (inducer off) you should measure 24vac. Once the inducer starts and is running you should measure 0 volts.
 
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Old 12-04-15, 05:50 PM
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Of the two pressure switches in my furnace, the parts list refers to one as the coil cover switch and the other as the blower switch. When I measured them with power on but nothing running, the blower switch read 1.3 and 25.8 and the coil cover switch read 0.0. After the furnace started working they read 24 when it was on and 0 when it was off, consistent with what you stated.

So I guess the problem had to have been a connection in the burner compartment, possibly combined with a sticking pressure switch?
 
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Old 12-04-15, 06:04 PM
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An intermittent connection is very likely. A sticking pressure switch..... pretty rare.
 
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Old 12-05-15, 02:45 PM
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http://www.goodmanmfg.com/portals/0/pdf/ss/ss-gmh95.pdf

The manual for this furnace can be found above.

It's apparently a two stage condensing furnace.

Presumably the "blower pressure switch" is verifying that there is good negative pressure.

The "front cover pressure switch" I don't recognize with any confidence. I'm guessing that the inducer motor may have a higher speed for the hi BTU input stage and that pressure switch is verifying that the higher negative pressure is being reached.
 
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Old 12-17-15, 02:27 PM
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Thanks a lot for your help guys! It's still running like a champ.
 
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