Method for monitoring a fuel cell system having a reformer for producing hydrogen-containing process gas from a hydrocarbon-containing combustion gas and oxygen and/or water vapour, wherein a combustion gas line and at least a first line which guides oxygen, preferably an air line, and/or water vapour line lead to the reformer, having at least one fuel cell stack, wherein the fuel cell stack is connected to the reformer by means of a connection line, wherein a second line which guides oxygen, preferably an air line, leads to the fuel cell stack, and wherein the fuel cell stack has a large number of fuel cells having anodes, cathodes and an electrolyte, having an electrical output line of the fuel cell stack and having a catalytic afterburner which is connected to the fuel cell stack by means of an anode discharge line and a cathode discharge line, characterised by the steps of - measuring the volume or mass flow in the first line which guides oxygen by means of a first mass or volume flow measurement device, - measuring the volume or mass flow in the second line which guides oxygen by means of a second mass or volume flow measurement device, - measuring the volume or mass flow in the combustion gas line by means of a third mass or volume flow measurement device, - measuring the current or the power in the electrical output line by means of a current or power measurement device, - calculating the oxygen supply at the afterburner using the measurement values, - calculating the quantity of oxygen required for a stoichiometric combustion using the measurement values, - calculating the air ratio as a relationship between the actual quantity of oxygen and the quantity of oxygen required for a stoichiometric combustion, - calculating a minimum air ratio permissible for compliance with the permissible maximum temperature of the afterburner, - comparing the actual quantity of air with the minimum permissible air ratio, - adjusting the supply of oxygen and/or fuel and/or outputting a warning signal and/or switching off the fuel cell system when the actual air ratio is less than the minimum permissible air ratio. - measuring the volume or mass flow in the first line which guides oxygen by means of a first mass or volume flow measurement device, - measuring the volume or mass flow in the second line which guides oxygen by means of a second mass or volume flow measurement device, - measuring the volume or mass flow in the combustion gas line by means of a third mass or volume flow measurement device, - measuring the current or the power in the electrical output line by means of a current or power measurement device, - calculating the oxygen supply at the afterburner using the measurement values, - calculating the quantity of oxygen required for a stoichiometric combustion using the measurement values, - calculating the air ratio as a relationship between the actual quantity of oxygen and the quantity of oxygen required for a stoichiometric combustion, - calculating a minimum air ratio permissible for compliance with the permissible maximum temperature of the afterburner, - comparing the actual quantity of air with the minimum permissible air ratio, - adjusting the supply of oxygen and/or fuel and/or outputting a warning signal and/or switching off the fuel cell system when the actual air ratio is less than the minimum permissible air ratio.