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I've run into a problem with my Goodman GMH95 furnace not igniting...

Background: At the beginning of summer, the capacitors in my AC condenser outside caught fire. It roasted the wires in the condenser, shorted out the compressor and fried the control board in my furnace. I replaced the AC condenser and the control board in the furnace. The AC has been working great all summer.

Now we've had a few cooler nights and it (finally) occurred to me that I had not checked the heat. So I turned the thermostat up. Guess what? No heat. I noticed a char mark on the connector for the gas valve and replaced the gas valve yesterday. Still no heat.

Here's what happens: 1. I turn the thermostat up to call for heat. 2. The draft inducer fan starts. 3. The pressure switches both close (one on the blower box, one on the coil? cover). 4. The igniter starts glowing and glows for about 10 seconds. 5. After about 5 seconds, the gas valve receives 24V on the orange wire for about 3 seconds while the igniter is on. I do not hear any click or smell any gas from the gas valve. 6. The igniter turns off. 7. The blower runs for about 15 seconds longer. 8. The blower shuts off. 9. Repeat steps 2-8 two more times.

I checked the pressure switches by applying vacuum and monitoring the resistance across the sensors. Both close at very low vacuum. I can also hear both close when the draft inducer fan turns on.

I cleaned the flame sensor for good measure.

I'm wondering if I got a bad gas valve. It was an eBay purchase of a (supposedly) new part. I imagine it's possible I got a bad one.

What else should I check before I buy another gas valve?

  • Is the voltage at the gas valve sagging? Have you measured it with a voltmeter, or tried jumpering R to W at the thermostat for that matter? – ThreePhaseEel Sep 4 '17 at 15:04
  • Not sure what you mean by "sagging." The voltage to the gas valve is 0 until after the igniter has warmed up. Then the board sends voltage to the gas valve (measures at ~27V). The voltage remains for 2-3 seconds when the igniter also turns off because a flame has not been established. – jrwca Sep 4 '17 at 18:19
  • is the voltage staying at 27V while the gas valve coil is energized, or is it peaking at 27V then dipping down? – ThreePhaseEel Sep 4 '17 at 18:35
  • It stays at 27V for 2-3 seconds. Then it drops when the igniter shuts off. – jrwca Sep 4 '17 at 18:44
  • can you try setting up your meter as an ammeter and putting it between the orange wire and the gas valve, then posting back the readings you get? – ThreePhaseEel Sep 4 '17 at 19:22
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If you are getting voltage to the meter but no gas it is either a bad gas valve or no gas. Check you have gas supply. If it were the pressure switches you would not trigger the igniter. If it were the flame proving system it would light and then go out after 2-3 seconds. If it were the circuit board you wouldn’t get 27 volts at the gas valve. No gas or very low volume or bad gas valve. I have seen where a clogged gas pipe would give me pressure but as soon as the gas valve opens the pressure drops to nothing, restricted line.

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I had the same problem. Make sure you have a Goodman hot surface igniter and not a hot rod or other universal igniter.

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