My 2005 Volvo XC70 Diesel has for the last four years intermittently given an "Engine Service Required" amber warning – error code P0244. Although the performance is slightly impaired, the light usually clears upon accelerating hard and the problem disappears. Neither my previous main dealer nor my local Volvo workshop has cured it so far and I’m not sure how I can fix it. The car is ideal for my requirements and I want to keep it for some years longer, but I’m worried this fault will cause a serious problem later on. What strategy can I suggest to my independent specialist workshop to help them sort it?