Air blower runs, inducer trys to start, all green lights board - maybe limit switch?

[tbone56]

I have a RUUD Silhouette II furnace and the other day the heat worked great in the morning but when I turned it on that night, only cool air blew out. I went to check it out and found only the air blower to be running and running. The unit would not respond to any thermostat settings. I cut the power, turned it back on the air blower just starts right up. When I do turn it back on, the draft inducer tries to start and then turns off immediately. Then the air blower just runs until I cut the power.

I think i have figure it out to being a limit switch problem, but am unsure because of my trouble shooting and the units board has two green lights that stay lit solid.

My unit has 3 manual reset limit switches next to the burners and one limit safety switch in the heat compartment. I checked the safety limit switch and it measures 0 ohms, so that is good. I then messed around with the 3 manual reset switches.

First question; on the manual reset switch, the tab on the back should be pressed in towards the middle causing the sliver disc the be pushed out and "locked" in place? I think that is right because did put a meter on it and it read 0v that way.

When all three of the switches are open (circle disc popped in and the tab is out away from middle) and I power up the unit, I can hear power getting to it but nothing turns on. Then when I reset just 1 manual switch, the fan turns on and the draft inducer tries to start. At one point I had the fan running and the draft inducer running, but the unit did not turn on. Then it went back to just the fan.

I have cleaned off the silver temp/flame sensor bar that sits in the flame, checked for loose wires, checked the hot surface ignitor.

Do you guys know what I can do? Any way I can bypass the limit switches to rule out anything else as being bad? Could it be the pressure switch? How can I check that?

One other note, the door interlock switch doesn't turn off the unit. The unit still does as I have described above. I pressed it several times but nothing. No big deal I guess.

And I do know that this is not a DIY site. I like to know what the problem is so that I can discuss it with a pro, and won't be doing any of the work. I learned to do this last time when I had a pro out right away and ended up buying a $300 filter. Thanks for your time.

[BaldLoonie]

Unfortunately since you acknowledge that this isn't a DIY site, you know we can't help you. I don't expect it to be a $300 filter but I'm not 100% sure what it is without getting in there with a meter. Which is what you need from a good local tech.