Switch the corresponding circuit breaker to the "Off" position to turn off the electrical supply to the furnace.
Locate the control door on your Rheem furnace. This is the top front panel on upflow models, the bottom front panel on downflow models and the right front panel on horizontal models.
Grasp the grab-hole on the furnace control door. Lift the door straight up and pull it toward you to clear the retaining clips on the sides and bottom of the furnace cabinet.
Lean the removed control door against the side of the furnace while you complete your troubleshooting or repairs.