In general, ultracapacitors do not have a hard end-of-life failure as do batteries. Their end of life is defined as when the capacitance and/or ESR has degraded beyond the application's needs.
Ultracapacitor failure can be caused by placing ultracapacitors in "abuse" conditions, which include over-voltage, over-temperature, and mechanical stress conditions. Over-voltage and over-temperature conditions can result in a loss of capacitance, increased ESR, bulging, and venting. Mechanical stress conditions can result in deformation, broken leads, and increased ESR.
Failure under Abuse Conditions