DEF is required on engines exceeding 74hp. What year was the requirement installed? Emissions are the biggest reason for equipment repairs in the largest skid steers.
Tier 4 Interim was 2008 approx.
Tier 4 Final was 2015 approx.
Both engines use regeneration fuel burning process along with a soot collector DPF muffler.
Final4- also added DEF//SCR as a NOX reduction system.
Both systems are problematic, require maintenance, and have both passive and stationary regeneration burn procedures.
What are you looking at?