This work proposes a temperature and irradiance sensorless diagnostic method suitable for small residential PV installations, focusing on detection of partial shadows. The method works by detection of failures in crystalline silicone PV arrays by concomitant monitoring of some of their key parameters such as series resistance, fill factor, and equivalent thermal voltage. These parameters show different deviations from normal values in presence of a failure - or, changes in environmental conditions. Using this information, it is possible to differentiate environmental effects from failures.