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My Furnace (a Rheem Criterion II Gas Furnace - model RGLH-07EAMGR) has had a problem for a while that we've just been living with but I can finally afford to do something about it.

What happens is that it will run fine for a while (hours, days, even weeks -- pretty random), and then it will stop running. I go out and check and the 'OK' LED is flashing 3 times and then off, 3 times and then off. The troubleshooting guide says this means "The limit switch is open".

What I have figured out is that if I wiggle the wires that attach to the pressure switch just a little bit (picture below), then put the covers back on, the furnace will turn on again and run normally for a while. Then, at some random time in the future, it will shut off and be back on the 'OK' LED flashing.

enter image description here

Based on my research, it sounds like Pressure Switches are usually an indicator of a problem rather than being the problem itself. But since wiggling that wire seems to reset things, I wonder if in this case it is the Pressure Switch? Or is it possibly actually the Limit Switch that needs replacing (I've never actually tried wiggling those wires)? What's the best way to troubleshoot this?

I will probably end up calling in a pro to take care of this, but I mostly want to understand so I can make sure I'm not getting ripped off..

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    If wiggling wires makes a different, it sounds like a bad connection or a bad wire. – keshlam Sep 27 '14 at 20:22
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    But in wiggling the wire, your probably also wiggling the sensor a bit, so it may still be the sensor. – Grant Sep 28 '14 at 13:58
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    Replaced the pressure switch, all is well now. – zipquincy Nov 17 '14 at 16:01
  • Maybe clean the contacts? The contacts may have dirt on them. – Fwappy_Potato Oct 10 '16 at 21:16
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It could be the connection or the sensor.

If those connectors move a lot when you wiggle them, they may just be loose, and vibrations from the furnace affecting them. So the first thing I would try is pulling the connectors off and squeezing them a little bit with a pair of pliers, so they dig in a little tighter when you put them back on. That costs nothing, so its worth a try.

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I had same issue with the pressure switch randomly shutting-off. I couldn't get it to fire back up until I would smack it. After looking up the four flash code and reading this blog I located the wiring of the pressure switch and found it was loose. After re-securing the wire it functioned properly.

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I have seen the same problem with my York furnace. I squeezed in the connectors and now it is working OK.

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    I presume you're talking about a York furnace, but more detail please. How did you squeeze the connectors: your fingers, pair of pliers, etc? A photo would help too. – Niall C. Mar 25 '15 at 23:46

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