Abstract: A high-temperature polymer electrolyte membrane fuel cell stack has at least two high-temperature polymer electrolyte membrane fuel cells (HT-PEFC), which comprise at least one means for determining the CO2 concentration of the cathode waste gas. A method for checking this fuel cell stack is thus possible, wherein at least the CO2 concentration of the cathode waste gas is determined. Optionally, the CO2 concentration of the oxidizing agent that is supplied can also be determined. The CO2 concentration is advantageously conducted in the region of individual cells or cell sections. This can be done, for example, by way of a displaceable lance in the cathode waste gas channel or a plurality of outlets (valves) in the cathode waste gas channel. If a threshold value defining the normal state of functional individual cells is exceeded, appropriate measures can be initiated, such as switching off the entire stack and optionally replacing cells or cell blocks.