Cats bacame mandatory on K plates if i recall correctly.

Most cars will have a air and water temp compensation.

If the engine is too cool it will give extra fuel, same as a choke.
If the engine is to hot it will give extra fuel as the fuel will help cool the intake and cylinders.

Fans running can draw power from other components such as injectors, coils etc and can affect the running of the car.
Most cars when the fans cut in will "skip a beat" this is due to the power being taken by the fans and the spark etc not being as strong as it should be.

Matt.