The ageing behaviour of electric double layer (EDLC) supercapacitors during over-temperature and over-voltage stresses is described. Specific experimental tests were carried out in order to reproduce, under laboratory controlled conditions, the above-mentioned stresses. The obtained results are discussed and interpreted to define indicators that distinguish between over-temperature and over-voltage faults occurring in these devices. For each stress type post-mortem examinations of the EDLCs were performed to observe any specific physical damage. These examinations have been used to define the fundamental assumptions of the proposed fault indicators. Three fault indicators were identified and related to the evolution of the following SC parameters: (i) the equivalent series resistance, (ii) capacitance and (iii) decay voltage during the relaxation phase. Additionally, new accelerated ageing and recovering phases were detected during combined high-pulsed power cycling and life endurance stresses.