To help avoid confusion, a lot of vehicles have a blue fill cap for the AdBlue tank and black. red or green for diesel, and sometimes on larger vehicles such as HGVs. the AdBlue and diesel fill points are positioned quite far away from each other. However, this isn’t foolproof and a driver mistakenly putting AdBlue into the diesel tank can have disastrous consequences.
Generally speaking, accidentally filling diesel into your AdBlue tank is unlikely as the AdBlue filler cap is designed for a smaller spout nozzle (19mm), making it impossible for larger diameter diesel nozzles to fit. The other way round however has become a common problem amongst drivers of vehicles with new SCR systems and causes costly damage.
If the unfortunate situation arises that a driver has added AdBlue in the diesel tank. Do not start the engine as this will circulate the fluid around the vehicle’s fuel system. AdBlue is not compatible with many metals and materials that make up the internals of the fuel system. This including cast iron, steel, brass, aluminum and zinc, so it can corrode the pipework and components. This can cost thousands of pound worth of damage, not to mention the loss of use of the vehicle until the tank is drained and all repairs have been carried out. You need to get a professional to drain the tank and flush out the system before starting the vehicle.