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The diesel exhaust fluid tank run dry, or filled with something that is not DEF. It is distinct from dilution: this is an absence or a substitution rather than a weakened solution, and it reaches the inducement stages faster because the system can detect it immediately from level and quality sensing.

Diesel put into the DEF tank is the expensive version of this mistake — the filler necks are deliberately different sizes to prevent it, and it still happens. Do not run the engine if the wrong fluid has gone in; the pump and injector are damaged by circulating it, so a drain before starting is far cheaper than a system replacement afterwards.

A tank allowed to run empty is usually a warning ignored rather than a surprise, since the vehicle counts down in miles for a long way first. Refilling clears the countdown, but on many vehicles the system needs a drive cycle to verify itself before the restart lockout lifts, so a refill in a car park does not always start the engine straight away.