Thermostat/AHU Issue

[tinmanwpk]

My AHU has been running continuously 24/7. I removed the thermostat from the wall as it is easily removable for backup battery access. The AHU continued to run. I changed the batteries. The AHU continued to run. The t-stat was then manually operated to complete shutdown mode and the AHU continued to run. The only way to shut the AHU down is to pull the cutoff next to it.

Any suggestions as to what the issue may be?

 

[BrandonV]

Stuck contactor/relay would be my first guess.

 

[PoorUB]

Electric heat? Stuck sequencer.