What will happen to a diesel vehicle if the DEF is not refilled?

When a diesel vehicle fails to have its Diesel Exhaust Fluid (DEF) refilled, the system is designed to ensure compliance with emissions standards. DEF is crucial for reducing nitrogen oxides (NOx) in the exhaust through a process called selective catalytic reduction (SCR). If the DEF level drops too low, the vehicle's onboard computer will trigger restrictions to prevent operation, leading to a disabling effect.

This happens as a safety mechanism to force compliance with environmental regulations. Once the DEF reservoir is empty, the vehicle might enter a 'limp mode', reducing its power and speed until the fluid is replenished. In some cases, the vehicle may become completely inoperable until the necessary fluid is added, ensuring that emission requirements are met. Hence, without DEF, the vehicle indeed becomes disabled rather than improving performance, consuming more fuel, or producing additional emissions.