If a driver ignores the dash warning message to refill a diesel vehicle with diesel exhaust fluid (DEF), what will be the end result?

Ignoring the dash warning message to refill a diesel vehicle with diesel exhaust fluid (DEF) can lead to the vehicle being disabled. Diesel vehicles equipped with selective catalytic reduction (SCR) systems require DEF to reduce harmful nitrogen oxide (NOx) emissions. If the DEF level gets critically low, the vehicle's onboard systems will detect this and activate safeguards, which can limit engine power and eventually lead to the engine not starting until DEF is replenished. This is a preventative measure to ensure compliance with emissions regulations and to protect the exhaust system.

The other options relate to various engine issues that are not directly caused by neglecting to refill DEF. While low DEF does put the emission system at risk, it doesn’t directly cause mechanical failures such as an engine lock-up or overheating scenarios. Therefore, the primary consequence of ignoring the DEF warning is that the vehicle will be rendered inoperative, necessitating a refill to resume normal function.