Repeated pressure switch failures

We have a Ruud Achiever, I think model R92P. It is almost ten years old and performed perfectly for the first nine years.

Last winter, we called a service tech out three times. All three times, they replaced the pressure switch. The last time, the tech said he used a different brand in case there was something about the original brand that wasn't working with our unit.

Two days ago, we had the furnace stop with the same code - 45 (pressure switch open). Called the service company again, tech advised us to turn the furnace off and back on, which got it working again. He said to keep an eye on it and advised us to call back during business hours to schedule a non-emergency visit.

We've noticed the furnace stopped four times total over the last two days, always with the code 45. When I turn it off and then back on, I get a variety of codes - 45, 46, 456, 4567, and 12, in a different order and combination every time - before it settles on H and starts heating. It has always started blowing warm air within a minute of being switched off then on.

It seems really unlikely that we could have four pressure switches go bad. Any ideas on what the root cause of our problem could be?

 

First thing to do is call a different company. No way they got 4 bad switches. Most likely the problem is in the venting or inducer blower.

 

Last year, our furnace started making a vibrating metallic noise when running (I think only when running at it 'high heat' mode and not in 'low heat' mode). It sounded like a fan hitting its housing.

On Saturday, I decided the noise had started bothering me enough (really, it was quite loud) that I went down to the basement to investigate. It seemed to come from a supply duct, and stopped when I put my hand on the duct. Something soft on the duct might stop it, I thought: I cut off the leg of an old pair of blue jeans and secured it around the duct with binder clips. No dice. Pressure, I thought - add a weight to apply pressure. A crescent wrench was handy, about two pounds, I hung it inside the blue jean leg. The rattle continues. Different positioning, maybe? I moved the wrench on top of the duct, and it wedged against the basement rafter. Bingo.

It was only days later that I realized the pressure switch errors - which had been happening at least twice a day since this latest cold snap started - have completely stopped. I believe the vibrations in that supply duct were jimmying the pressure switch stuck. How strange is that?