"Pressure Switch Stuck Closed" - Running out of Parts to Replace!

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  #1  
Old 05-10-13, 06:27 PM
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"Pressure Switch Stuck Closed" - Running out of Parts to Replace!

Hi,

I'm vexed and could really use some help.

* Amana 90 Air Command Gas Furnace
* ~12 yrs old, Has Been Working Great
* White-Rodgers 50A65-288 Controller

Noticed the house was cold and the furnace wasn't running. Reset the furnace electrical switch and it consistently starts to operate (fan noise, element gets red hot, flame ignites) and then a few seconds later shuts off.

Red LED blinks 2 times and it tries again. The label on the door indicates this is a "Pressure Switch Stuck Closed" condition.

Removed upper and lower panels. Visual inspection shows nothing. No soot/burn marks, loose wires, leaking anything, cracked hoses, etc. I'm no expert but nothing seemed wrong.

The condenser trap was gunky so I removed it, fully cleaned it and re-installed it. Same problem.

Even though my blow/ohmmeter test indicated both pressure switches were fine, I replaced them both anyway. Same problem.

Then I replaced the draft motor, which was spinning fine until the flame comes on and then everything shuts down. Same problem.

Then I replaced the controller board. Was a 50A65-288, now the universal replacement is a 50A65-843. Wires matched up perfectly. Same problem.

Then I removed all of the hoses (2 to pressure switches, 2 drain, I think) and made sure they were not blocked. And I ran a fish-tape from the outside vent 25' in, which I think would reach where the vertical vent from the furnace reaches the ceiling - the bend is too sharp there to fish past. And when I replaced the draft motor I could feel the cold outside air on the vent tube - so I don't think the vent to outside is blocked.

When I put a voltmeter on the "Panel" pressure switch it changes voltage during the start-up sequence. When I check the "Blower" pressure switch it reads +24V during the entire start-up sequence. This supports the diagnostic code, I think. I'm guessing the controller expects it to drop to 0V at some point, probably when the flame comes on.

I'm running out of parts to replace! The only things I see left don't seem relevant to the diagnostic code:

* Ignitor?
* Flame Sensor?
* Capacitor?
* Circulation Fan?

When I troubleshoot I leave both panel doors off and manually hold the panel-safety switch closed. But I get the same problem when I put the panels back on.

I could really use some ideas on what else to try... I'm well past my expertise level on furnace internal operation.

Thanks for any suggestions!

Regards,

Ryan
 
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  #2  
Old 05-10-13, 06:35 PM
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I don't see where you cleaned the flame sensor and your description sure sounds like a dirty flame sensor. Remove and clean it with fine steel wool. Just clean it, not wear it away and see if that cures your problem.
 
  #3  
Old 05-10-13, 06:52 PM
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Ah ha!

I gave the flame sensor a little fine grit sand paper (no steel wool handy) and that resolved the problem!

I guess I was so fixated on the "Pressure Switch Closed" diagnostic code... I initially saw that there was a different code for the flame sensor and didn't go down that path at all.

*THANK YOU* !

Hmm... 8 minutes from my post to your fix. I'd say that's awfully impressive.

Regards,

Ryan
 
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