The fault detection applied to a large amount of small distributed PV systems needs to be simple, cost-effective, and reliable. This work presents a fault detection procedure applied to distributed PV system fleets, based on a novel performance indicator, designated as Performance to Peers (P2P), that can be constructed on the sole basis of the comparison of the energy production data of several neighboring PV systems. This article explains how to construct this performance indicator and how to use it to carry out automatic fault detections. This fault detection procedure has been developed in the context of the performance analysis carried out on approximately 6000 PV installations located in Europe, and it is illustrated and discussed through real application cases. The P2P has been shown to be more stable than the Performance Ratio (PR), in particular in the presence of sub-par metadata on the PV systems, and it thus constitutes a more robust performance indicator for fault detection. The stability of P2P is characterized by an Absolute Median Deviation (MAD) that is typically of 10% for hourly data and 5% for daily data. The application of P2P to fault diagnosis is illustrated on four categories of faults that are among the most frequently observed on PV systems. The main limitations of this novel methodology are discussed, and several future lines of research are suggested.