I have an Audi S4 (the 2004 B6 version). The cause of the engine warning light coming on has been diagnosed by my local Audi dealership as a failure of the coolant sensor. They have asked for £310 to effect the repair. As the part is £34, they are allowing 2.5 hours labour at their prevailing rate. This seems excessive to me, given the location of the sensor - near the top of the back of the engine. I have a mechanic friend who I would trust implicitly to carry out the replacement, at a much reduced cost to me. However, will the replacement of the sensor automatically clear the warning light, or will I need to go back to the dealership to get this done?