of monitoring the lambda values of each exhaust bank the ecu 9 monitors the lambda correction values for each bank works out a relationship x 1 between the values prior to a switching event computes a relationship x 2 between the same values subsequent to the switching event then determines a third relationship x 3 between x 1 and x 2 if x 3 is found to fall within a predetermined range then the ecu deduces that degradation exists in the cps system of the switched bank a degradation signal is then logged by the ems 10 and displayed on the instrument panel as in the embodiment described above with reference to fig2 the relationships computed may be ratios alternatively the relationships can be a difference between two values in a preferred embodiment the sensors for monitoring air fuel ratio may be lambda sensors the output of such sensors is a value which is equal to the actual air fuel ratio divided by the stoichiometric air fuel ratio the stoichiometric afr defines the minimum theoretical mass of air necessary for complete combustion the description can be embodied as a monitor which operates by causing the deliberate switching of cam profile on one bank and assessing a change in the relationship of.