I have a 2 year old 4 ton American Standard HVAC unit and the blower fan will not turn off no matter what I do. This problem happened at the same time last year and the tech cam out and replaced my thermostat and all was fine. I have troubleshooted the thermostat and this is not the problem. I have reviewed other posts online with similar problems so I went into the attic and removed the panel cover to find that the code light was blinking 4 times. It states on the panel cover this means the limit switch is to blame. Where is the limit switch and what does it look like? I think I know what it is, but not sure. I located a button type part with 2 screws and 2 wires attached to the blower, is this it? I removed the wires and removed it to see, but nothing that helps me id it for sure? Any help?

You do have a limit switch open---it could be a roll out, blower or main limit open. The limit switch will have 2 wires going to it, round disk. The button in the center is the reset button. If you push it and hear it click that one was tripped. That's a manual reset switch. Some will reset after temp drop or power has been turned off and then restored.