Well Chris, simple things eh, that would actually get the voltage reading up at the lambda sensor slightly as would detect less oxygen. Have used one before on a near standard Subaru Impreza to fool the ECU as it ran a high flow cat and would whack it to limp mode every 250-300 miles. Was sceptical at first but it did seem to do the trick and would go through the MOT with only 1 cat fitted.