When I arrived, I had two different error codes showing up for diagnostic. I was able to clear the pressure switch code and cleaned out pressure switch ports on each end. I was able to close the pressure switch. Lastly I had a high limit switch that prevented unit from turning on. I checked both high limit switches and received continuity on both. I found a dirty air filter and replaced it because restriction can cause limit switch to trip. After failing to get system on, I turned on cooling for 10 minutes to cool switches and system down. I turned back onto heating and same issue. The ducting intact to system needs to be replaced and undersized. System will turn on after it cools down but problem will consistently happen when temperatures get too hot. System is operational upon departure. System is ten plus years and needs replacement along with ducting to work efficiently in this commercial space.