Which statement best describes the consequence of not keeping the DEF tank filled?

• A. Engine derating and could limit speed
• B. Reduced fuel economy
• C. No effect
• D. Higher top speed

Answer: (A)

Not keeping the DEF tank filled triggers the vehicle’s emissions-control protections. Diesel Exhaust Fluid (DEF) works with the selective catalytic reduction system to lower NOx emissions. When DEF runs low, the aftertreatment system can’t function properly, so the engine is derated to limit power and protect the exhaust system. That derating often comes with a restriction on top speed until DEF is replenished. This is why the consequence described is engine derating with a possible speed limit. The other options don’t fit because empty DEF doesn’t increase speed or improve fuel economy, and vehicles will typically warn you or enter a reduced-power mode rather than having no effect at all.