[hoadle]

My sister bought the house about year ago, last week she toll me The circulation blower does not turn off –always running (Trane XE90 gas furnace units in the house)
Although the thermostat is set on 'Off' vs. 'Heat' or 'Cool' and fan ( I mean circulation blower) is on 'Auto', the fan runs constantly.
The sight glass on the furnace shows a red light is off.
(Look like gas furnace & air condition unit is working final only problem the circulation blower is run continuous)
I disconnect the Thermostat control of base the circulation blower still running is tell me the Thermostat control not control the circulation blower
I check HPS/PS2 (High Limit SW) & LPS/PS1 (low limit SW) + inducer housing limit SW. All had continuity (normal close switch)
I check Vacuum switch is no continuity (normal open switch) when I turn on the heater & check vacuum switch is had continuity.
I switch to air condition (unplug blue wire for heater circulation blower) is circulation blower turn on about 30 second & shut off (I hear 2 cliks at relay one begin & 30 second late is turn off the circulation blower.
I can’t find the Trane XE90 gas furnace manual online but I folow the dirgram at housing door check all thing above. Maybe I missing some step from Troubleshooting to check out the circulation blower setup? Are Trane XE90 had fan limit switch where local?
But I know some one replace new control board 2 years ago
Please let me know what wrong my sister