Volkswagen Crafter abs sensor

ABS light on for a year, finally get round to check each sensor, poor continuity on rear, two new sensors fitted to rear of vehicle, got resistance readings three of wheels match drivers side rear has a different reading. No warning lights when you first drive off, first ABD light (traction warning) starts flashing then ABS light comes on and stays on! Senor gaps are all the same and new sensors on rear. Does the bosch system need calibrating because sensors have changed?