An apparatus including an array of sensors including a plurality of chemical sensors and a plurality of reference sensors, each chemical sensor coupled to a corresponding reaction region for receiving at least one reactant, and each reference sensor comprising a field effect transistor having a gate coupled to a corresponding reference line and an access circuit for accessing the chemical sensors and the reference sensors and a controller to apply bias voltages to the reference lines to select corresponding reference sensors, acquire output signals from the selected reference sensors, and identify one or more defects in the access circuit based on differences between the acquired output signals and expected output signals.