To balance ozone sensors for the onboard diagnosis of a catalytic element which is arranged in a vehicle and is exposed to an ambient airstream, for breaking down ozone, predetermined enable conditions allowing the sensor balancing to be carried out are checked, and if the enable conditions are fulfilled the values of the output signals from the ozone sensors are recorded and compared with one another. This comparison gives a deviation value which is characteristic of the deviation between the two values, and the deviation value is used to decide whether sensor balancing is required and indeed possible. If sensor balancing is required and possible, the values of the output signals from the ozone sensors are compared with a threshold value and depending on the result of this comparison either an additive or a multiplicative correction of the values of the output signals is performed.