AdobeStock 209046721North Americans that rent cars in Europe have a good chance of being assigned a vehicle with a diesel engine. Everybody is aware that at some point a car’s fuel tank will run low and require a fill-up. In fact, there's a gauge on the dashboard that keeps track of the fuel level.  

However, there is another product that's required to keep European diesel engines running. It’s a liquid additive called AdBlue and when it runs low it adversely affects engine performance. When it’s gone the engine will no longer operate. Though there is an AdBlue dashboard warning light there is no gauge to track its level. Typical AdBlue consumption ranges from about one liter per 350 to 600 miles. 
So, when the agent at the rental counter hands you the keys to a diesel engine car make sure the AdBlue level is full, just like the fuel tank. 
If, during the rental, you must replace AdBlue, you can do so at most service stations. AdBlue is usually refilled next to where the fuel tank is refilled and its cap is blue. Most AdBlue tanks hold about 10 liters so replenishing will probably not be required of most renters. If you must do so, however, keep your receipt as you can probably be reimbursed by the rental company.