An operator performs a pre-shift inspection on a piece of heavy equipment in -15°C weather. The DEF tank is full, but the DEF warning light is illuminated. What is the most probable cause, assuming the DEF system was properly maintained?

Question: An operator performs a pre-shift inspection on a piece of heavy equipment in -15°C weather. The DEF tank is full, but the DEF warning light is illuminated. What is the most probable cause, assuming the DEF system was properly maintained?

Answer options:

• The DEF system is contaminated with incorrect fluid.
• A sensor in the DEF tank has malfunctioned due to prolonged cold exposure. ✅ The DEF fluid has solidified overnight, and the system is still in a thaw cycle.
• The DEF fluid concentration is incorrect, triggering a warning.

Correct answer: The DEF fluid has solidified overnight, and the system is still in a thaw cycle.

Explanation: At -15°C, DEF freezes. Even with heating elements, it takes time to thaw the entire tank. The warning light likely indicates the system is still in a thaw cycle and not yet operational, as per manufacturer guidelines.