Hi - we have a Trane XR80 and just tried to turn it on for the first time this year. The thermostat clicks on and the fan blows for a minute and then turns off. We removed the panel and a red light blinked three times, which, according to the trouble shooting codes means a pressure switch error. We replace the filters once or twice a year and put a new filter in before we tried turning it on tonight. How do we find the switch (the control board diagram is frightening)? And how do we replace or reset? Any info is helpful. Would love to fix on our own, if it's a simple fix, rather than waiting for service. Thanks.