Arrived at the customer’s home to find the cause of the issue with the upstairs HVAC system. The customer received an error telling them that the system was turned off after a low pressure lockout was initiated (error code 83). Tested heating from the thermostat. Measured the refrigerant pressure and electrical component readings. Inspected the safety and cleanliness of the system. Checked and replaced the 4” media filter with one provided by the customer. Ran duct load calculations and measured the static pressure. It appears that the system was switched into cooling while it was cold outside, causing the low pressure lockout. Reported findings to the customer.