ENGINE MAINTENANCE - Maintenance Procedures 2 1. Replace the aftertreatment diesel exhaust fluid (DEF) dosing unit (DEF module) filter according to the maintenance schedule. 2. From the bottom of the tank, remove the cover plate by removing the various fasteners holding it on then allow the cover plate to drop down. Air System Air Intake System General information and precautions on how to maintain the air intake system. 74 Y53-1166-1G1 (08/2018) Medium and Large DEF Tank Small DEF Tank Filter Access