Warning lights on 1998 Audi A6

Recently the following lights come on after driving my car 15 minutes: ABS, Check Brakes and low oil pressure. My mechanic has already told me that The sensor is bad and that is why the ABS and check brake light comes on. Told me the brakes still would operate, but that I would not have the anti-locking or other computer generated functions that this sensor provides. He didn’t check for the oil pressure because I thought I was going to trade in car. All 3 of these lights always come on at the same time. Is it likely that the low oil pressure light is a bad sensor too. Why would they come on instantaneously? When these lights come on the RPM indicator drops to zero, When I accelerate the indicator does not move. Also if the air conditioning is on it suddenly stops blowing cold when this occurs. If it is only a sensor, am I causing any engine trouble by continuing to drive. I really appreciate any help concerning this question.