While driving in Greece recently, my 2005, 45,000-mile Volvo XC70 2.4 D5 flashed up 'Anti Skid Service Required'. On reaching home 1500 miles later, with no apparent problems apart from having this message constantly displayed, I took car to a main dealer. A computer systems test resulted in a bill for £260 and a quotation for supply and fit a replacement Differential Electrical Module at cost of £1513.

On leaving the dealership, I advised them that the warning sign had disappeared and the dash computer seemed to be working as normal. Had it in fact been fixed by diagnostic work? No, they said, it will re-appear in a few miles. 500 miles later, there's still no warning and the car appears to be OK. Do you have any knowledge of this type of issue? How does one test for correct operation of the 'operate on demand' four-wheel drive?

Discussions at the dealership revealed that during the winter snow, several XC70 and XC90 owners found that, unknown to them, their 4WD was not in fact working.