Fluids, Filters, and Lubrication
This domain tests whether you can select and service the right fluids and filters and recognize when condition calls for action. Know the major fluid families and their jobs: engine oil (lubrication, cooling, cleaning, sealing), coolant (heat transfer and corrosion protection), transmission/differential lubricants (gear protection), power steering fluid, brake fluid, and diesel exhaust fluid (DEF). Understand that specifications matter-using the wrong coolant or oil can cause cavitation, gelling, or additive dropout. Know the difference between conventional, extended-life, and organic-acid coolants and that mixing types is a trap.
Filters: engine oil, fuel, air, coolant, and transmission filters. Understand that a restricted air filter shows as high restriction (use a restriction indicator), not by looking dirty, and that over-servicing an air filter can be as harmful as neglect. Know fuel filter water separators require draining and that water in fuel is a common cause of hard starting and injector damage. Recognize that DEF has a shelf life and freezes; contamination is a service issue.
Traps: judging oil by color alone, topping off coolant with the wrong type, ignoring a fuel-water separator, and assuming a dirty-looking air filter is restricted. Another trap is overfilling oil or coolant, which causes foaming or overflow.
Rule of thumb: match the fluid to the manufacturer's specification, and check level on a level surface with the engine off unless the procedure says otherwise. Worked example: a truck loses coolant with no external leak. Pressure-test the system, check the cap, and verify coolant type-do not simply top off with water, which dilutes additive protection.