ASE A9 Cheat Sheet 2026

The 30 highest-yield ASE A9 facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. While inspecting a pushrod, a technician rolls it on a flat surface and sees it wobble. What does this indicate? → The pushrod is bent and must be replaced
2. Which valve spring measurement shows that a spring is bent or distorted rather than weak? → Squareness, checked against a square on a flat surface
3. After a valve job on a diesel engine, which measurement most directly shows whether the valve was sunk too deeply into the head? → Valve stem height or valve recession measured from the deck or the spring seat
4. Diesel exhaust fluid (DEF) used in SCR systems is made of which mixture? → 32.5% urea and 67.5% deionized water
5. Which tool is commonly used to verify that DEF has the correct urea concentration? → A refractometer
6. A diesel engine has coolant in the oil but no external leaks, and the head gasket has been ruled out. What is the best way to test the cylinder head for cracks? → Pressure test the head's coolant passages after plugging all ports, and check for leaks
7. At what point does diesel fuel begin to form wax crystals that can plug fuel filters in cold weather? → The cloud point
8. Engine oil is found in the coolant, but there is no coolant in the oil. The leak is at the engine oil cooler. What best explains the direction of the leak? → Oil pressure is higher than coolant pressure at the cooler
9. Which statement about an ethylene glycol and water mixture in a diesel cooling system is correct? → A 50/50 mix gives freeze protection down to about -34°F (-37°C)
10. After installing new piston rings, a technician hones a cylinder. What is the purpose of the cross-hatch finish left on the bore wall? → To hold oil on the wall so the rings can seat and lubricate properly
11. Which inspection method is best for finding a suspected crack in a cast iron diesel engine block? → Magnetic particle inspection
12. During a cylinder balance (power contribution) test, one cylinder shows almost no drop in engine speed when it is cut out. What does this indicate? → That cylinder is contributing little or no power
13. A thermostat removed from a diesel engine is being bench tested in a container of water. What indicates that the thermostat is good? → It begins to open near its rated temperature as the water is heated
14. A technician performs a crankcase pressure (blow-by) test and finds a reading well above specification at a steady rpm. What does this most likely indicate? → Excessive combustion gas passing worn piston rings and cylinder walls
15. Why is supplemental coolant additive (SCA) maintenance especially important in a diesel engine with wet cylinder liners? → It protects the liner outer surfaces from cavitation erosion
16. An oil analysis report shows elevated sodium, potassium, and boron, along with a drop in viscosity. What does this pattern most likely indicate? → Coolant leaking into the engine oil
17. A dial indicator is used to measure crankshaft end play. The reading is above specification. Which part is most likely worn? → The thrust bearing surfaces
18. A diesel engine cranks very slowly and will not start. The batteries are fully charged. What should the technician check next? → Voltage drop in the starter circuit cables and connections
19. What is the primary purpose of a pressure-rated radiator cap on a diesel engine cooling system? → To raise the coolant boiling point by pressurizing the system
20. Approximately at what temperature does standard DEF begin to freeze? → 12°F (-11°C)
21. A diesel's fuel supply contains a significant amount of gasoline from a mis-fueling event. What is the main danger to the fuel system? → Reduced fuel lubricity and cetane, which can damage the high-pressure pump and injectors
22. When a scan tool retrieves a stored DTC, what does the associated freeze frame data show? → Engine operating conditions recorded at the moment the fault was stored
23. A diesel engine emits blue-gray smoke, especially after idling, and oil consumption has increased. Which condition is the MOST likely cause? → Engine oil entering the combustion chamber past worn valve guides, seals, or piston rings
24. Where should a technician connect a manometer or restriction gauge to measure intake air restriction on a turbocharged diesel engine? → In the intake piping after the air filter and before the turbocharger compressor inlet
25. During an active DPF regeneration, what is the main purpose of the diesel oxidation catalyst (DOC) located upstream of the filter? → To oxidize injected fuel and raise exhaust temperature to burn soot in the DPF
26. Which finding most directly points to a failed compressor-side oil seal in a turbocharger? → Oil in the charge air cooler and intake piping
27. A diesel engine emits white smoke with a sweet odor, and the coolant reservoir level keeps dropping with no external leaks. Which is the MOST likely cause? → Coolant entering the combustion chamber through a failed head gasket or cracked head
28. A diesel engine has high oil pressure at cold start that drops to normal as the engine warms up. Which of the following explains this? → Oil is thicker when cold, so flow resistance and pressure are higher until it warms
29. What is the function of the bypass valve in a full-flow engine oil filter? → To let oil flow around a clogged filter element and avoid oil starvation
30. Why must a diesel cylinder head's thickness be measured before it is resurfaced? → To make sure removing material will not drop it below the minimum thickness specification