Volkswagen Polo knock sensor fault code

code reader listed fault with knock sensor, so replaced knock sensor and still problem is listed. 00524(g61) code.knock sensor-( open or short circuit to earth).to me that sounds like the wiring. any ideas. runs find around town,but if your on the open road within about 5 miles of travelling, it just loses all power and drinks petrol. if codes are continually reset on a journey of say 20 miles, its good on fuel, but go 20 miles without resetting codes and it can use between half a tank and 3 quarters.due to be looked at by a vw garage, just dont want to be ripped off .vw bloke says he`s never heard of the problem before on my make of polo. so its a new one on him.any advice how ever small ,would be great.