My son is thinking about buying (private sale) a 2011 Audi A4 Quattro, 99k miles on the clock. The only obvious fault is the AWD/Sports Differential warning light comes on (amber) when turning the car hard left. Seller says it's been like that for ages and never bothered about it. Hmm...The car drives perfectly, it's just the warning light that's cause for concern. Does anyone have experience of this warning light, likely cause, cost to fix etc? Any thoughts appreciated.