How to Troubleshoot a Duct Detector's Open Circuit

Establish the location of the duct detector by referring to the air-conditioning plan for the building or home.

Select the “Fan Only” function at the air-conditioning controls. Allow the fan to start.

Stand an A-frame ladder at the area of the detector. Remove the screws from the grill at the face of the wall with a nut driver. Remove the grill to access the duct detector. Skip this step if the duct detector is accessible in an attic.

Set a voltage meter on "Ohms." Pull the wire connectors off the electrical tabs at the photoelectric sensor. Touch one of the pointed tips of the meter wires onto each of the electrical tabs. Note the reading on the meter.

Refer to the Ohms for the duct detector as shown on the label or as specified in the air-conditioning plan. Typically, duct detectors are rated for 30 or 60 Ohms. Replace the detector if the reading is below the Ohms listed on the label or in the plans. Otherwise, reattach the connectors and proceed to the next step.

Select the “Volts” setting on a voltage meter. Touch the pointed tips of the meter wires onto the exposed portions of each electrical tab at the photoelectric sensor. Observe the reading on the voltage meter. The circuit is open if the voltage at the photoelectric sensor is “Zero” when the HVAC unit is running. If the Ohms reading is correct, the problem is in the electrical circuit to the detector.