If it is the Diesel Particulate Filter, you could try a DPF cleaner from STP or Redex or others. You can find them on Amazon. I recently used one as I kept interrupting my regen by doing short journeys. So I put the stuff in my tank, went for a 20 minute drive on the motorway, the regen completed and no problem since. Admitedly my warning light hadn't come on, but according to the reviews, a lot of warning lights were cleared by using these cleaners.