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.

 

I have a 2011 and never incurred this problem. In reviewing the owner's manual:

Anti-lock brake system (ABS)
The warning lamp monitors the ABS and the electronic
differential lock (EDL).
The warning lamp  lights up for a few seconds when the ignition
is switched on and while the engine is being started. The lamp goes
out again once the system has run through an automatic test
sequence.
There is a fault in the ABS if:
• the warning lamp  does not light up when the ignition is
switched on,
• the warning lamp does not go out again after a few seconds,
• the warning lamp lights up when the vehicle is moving.
The vehicle can still be braked in the normal way (except that the
ABS control function is out of action). Please take the vehicle to a
qualified workshop as soon as possible. For further information on
the ABS ⇒ page 222.
If a fault occurs in the ABS, the ESP warning lamp will also light up.

Fault on the electronic differential lock (EDL)
The EDL works in conjunction with the ABS. If a malfunction should
occur in the EDL, this is indicated by the ABS warning lamp.
Please take the vehicle to a qualified workshop as soon as possible.
For further information on the EDL ⇒ page 222.

From page 222:
Electronic differential lock (EDL)
When the EDL detects wheelspin, it brakes the spinning wheel and
directs the power to the other driven wheel (or wheels on quattro*
versions). This function is active up to about 100 km/h.
To prevent the disc brake of the braked wheel from overheating, the
EDL cuts out automatically if subjected to excessive loads. The
vehicle can still be driven. The EDL will switch on again automatically
when the brake has cooled down.

So, IMHO, the car is driveable with a problematic EDL but that is an important safety system if you live in a wet, snowy, or icy location. The problem could be in the ABS unit (which can be removed for rebuild and the car is still driveable) or it could be in a wheel speed sensor. Have the vehicle scanned for codes - may be helpful. I removed the ABS unit in my 2003 A6 and sent it out for rebuild. It was returned with "no problem found" and when re-installed everything worked normally (go figure). Other than that, no problem in this area on a 2003 A4 and current vehicle.