I've just had CEL P0303 15 minutes after fitting a 7 wire grounding kit. Car was perfectly ok before this. Dealer advised me that I may have a faulty spark plug - these are due a change anyway so will try this first and post back results.
Just cant help but wonder if, due to better grounding, one of the sensors is now operating more efficiently and is showing up a fault that would otherwise not have thrown a CEL
Mark