There are 2 oxygen sensors.

The first is at the exit from the turbo and has a green connector.

The second is at the beginning of the flexi-pipe. Its job is in fuelling control to speed up the cat light-up, and confirm cat health in normal running.

Its this second function that causes the MIL to be on after de-cat, either because its screwed in and doing its job, or because people just blank off the tapping and disconnect it.

The ECU still has to receive a signal from the second (blue) sensor. Having the signal passing through the MIL Eliminator creates enough fluctuation to make the ECU think there is still a working cat in the system.

£60 . I still make them for a £5 (2 PCBs ready to go). the expensive bit is wiring them in.

Martin