did another one of these yesterday--the cap must have 2 magnets in place & the sensors are failing quite often now,easy to replace just remove wheel inner wheel arch cover & undo filler neck,then replace sensor & refit,i used snap on modis to check live data for confirmation of cap on or off in diesel additive category,but for some reason the modis would not clear fault code in engine so had to use autodiagnos mt pro to reset,then was no problem.
hop this helps
colin
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