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Pressure switch is closed with Inducer off, tried several experiments. What next

Pressure switch is closed with Inducer off, tried several experiments. What next

Old 12-15-12, 02:41 PM
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Pressure switch is closed with Inducer off, tried several experiments. What next

I have a Gibson, GL1RA, forced air furnace. It is about 7 to 8 years old. For the last year, I had intermittent problem when it would not light up. When I turned off the power fully and turned on, it generally cleared the problem. But now, it does not solve the problem. Now when I turned on the thermostat, it turns on the inducer motor, then turns on the igniter. Igniter glows bright orange. But the gas does not light up. Then the igniter shuts down. After some time the cycle repeats. At the end, the control board light begins to blink. It is three blinks which per furnace document means "Pressure switch is closed with Inducer off". Note that inducer motor is running all this time.
I have done the following tests
1. With furnace off (i.e., power on but keeping thermostat below room temperature), the voltage across the pressure switch leads is around 4.5 volts.
2. With Furnace on, and both wires going to pressure switch taken out, the voltage between the two wires is around 24 volts. (I can't easily measure voltage when both wires are connected to the pressure switch and furnace on).
3. I disconnected both leads to the pressure switch. Then turned on the furnace. Control board immediately started blinking with 2 blinks meaning "Pressure switch is open with inducer on". When I shorted the two leads (I guess this is same as forcing the pressure switch to close), igniter came on but gas did not light. Then eventually ended up with light blinking three times.
4. I disconnected pressure switch hose and blew air to ensure that it is not plugged. It was easier to suck air then to blow into it.

What do I do next?
Old 12-15-12, 10:38 PM
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You already did most of the necessary troubleshooting. At a minimum the pressure switch needs to be replaced.

With the power off, check the pressure switch for continuity. Open is normal when the furnace isn't running; near zero resistance indicates that the switch is stuck closed.

As a general rule, never bypass a pressure switch with the gas on...

With the gas to the furnace off (so it doesn't light off without a proper draft), turn on the furnace and bypass the pressure switch a a second after the draft inducer fan comes on.

If the board is good, it will give a "flame not sensed" (or something along those lines) error code shortly after the igniter glows.

It's 24 volts so you won't get shocked - just don't allow any disconnected terminal to touch the chassis as to prevent the low voltage fuse from blowing.
Old 12-16-12, 12:35 PM
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Almost sounds like a control module issue. The module gave you two blinks as it should when the pressure switch was disconnected. You tried connecting it, ignitor activated and then three blinks. You should have gotten the 3 blinks before the ignitor activated. Once the ignitor activated it should have continued with the cycle.

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