What fault codes does it show?

If it's P0141 it's a hego sensor (lambda) heater circuit failure. If cat is inefficient it'll show a P0420, Ideally needs both sensors data logging to see if the output voltage from the post cat sensor mirrors the pre cat sensor output voltage. If cat and post cat sensor are good the output voltage of post cat sensor should be constant, not switching between 0.2 -0.8 volts when in closed loop.