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1001miles 10-19-2008 04:17 PM

Pressure switch stuck open? 2 areas im looking @
Hi, I've been searching for a few days & am unable to find what I'm looking for.

I have a 7 year old furnace. lennox ultra 5 advantage.

Went to turn on heat & nothing!!

The led trouble code is 3 blinks = "Pressure switch stuck open 30 seconds or longer after call for heat. on/off switch in "off" position during call for heat will generate this code ( this is right out of the furnace manual)

After researching I believe it is one of two items:

1. Draft inducer fan is bad.

2. Pressure switch is bad.

I cannot tell if the Draft inducer fan is actually spinning. I can hear the fan unit "hum" & it is very hot to the touch. However I am unsure if the fan is actually spinning!

I tested for continuity (w/ohms) from the black - green & white-green wires & found no continuity on both. There was continuity from black to white.

I am wondering how else I can test the fan to see if its working?

2. The tubes don't appear to be cloggeed, I blew in the one coming off the inducer fan & i can audibly hear the pressure switch clicking. Not sure how to test the switch with an meter.

Any thoughts / tips would be great. I'd like to try & figure this out without having to call in the repair man :(


glennjanie 10-19-2008 06:28 PM

Welcome 1001Miles:
We are happy to have you and hope we can be of help to you without costing the whole bank account.
Looks like your draft inducer fan motor is bad. You should be able to hear it running, especially if you put a screwdriver against the center of the motor and place your ear against the handle (its like a stethescope). The hum and the heat are my clues. If you have an ammeter you can check the amps on the power wire to the motor; if it is running it will show something like 2 or 3 amps, if its stuck it will show more like 6 amps. Its called 'locked rotor amperage'.
Also, if you have access to your exhaust pipe outside, it should blow a piece of toilet paper in your hand if the motor is running. The inducer motor is a high loss item on high efficiency furnaces.

kok328 10-20-2008 09:41 AM

break it down until you can get access to the inducer fan blades. nurse a little oil into the shaft bearings, spin it a bit by hand, reinstall and have warm and happy day.

1001miles 10-20-2008 04:35 PM

Thanks for both ur responses.

So itested white & ground (both tests while the inducer was "humming") & had .6 & on the blakc & ground was 120v.

SO sounds like I have a bad draft inducer fan. Is this something that a cleaning will fix or do I need to buy a new fan? Any idea what price range on new fan is?

Thanks again!!! :)

1001miles 10-20-2008 05:55 PM

soooo never guess what I found when I took out my draft inducer?

2 dead birds were stuck in it & the pvc tube!!!

Everything now works like a charm -

Thanks for helping guys!!

Square Eye 10-20-2008 07:43 PM

You might go to the local home center and find a strainer cover or a screen and cover that tube end to prevent that from happening again :)

falconphysics 11-02-2008 11:03 AM

Same Problem (not the Dead Birds)
I have the same problem. "Pressure Switch Stuck Open"

When I jumper the switch everything seems to work fine. I have good flow of air at the input and output vents on the outside of the house. The draft inducer fan seems to be spinning.

I pulled the switch and blew through it to see if the switch was closing. It does, checked continuity of the switch and it seems to be fine.

As far as I can tell, I'm left with a couple of choices here. The pressure switch could not be switching at the proper pressure or something else is not working.

Any help would be greatly appreciated.

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