Fault diagnostic strategies can be classified into off-line and on-line methods. Off-line methods involve experiments that take place under laboratory conditions and may require disassembly of the PEFC . While these techniques are especially useful in understanding the fundamental characteristics and behaviours of PEFCs, their intrusive nature makes them inappropriate for onboard, real-time use in operational cells. In contrast, on-line diagnosis techniques only use (often crude) signals from the sensors attached to the PEFC to estimate its state of health (SoH).