DTC Cheat Sheet 2026

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

1. What role does a glow plug play in a diesel engine? → Heats air for cold starts
2. Which of the following best describes a 'B-service' interval on a heavy-duty diesel truck? → A major service including oil, filters, fluid checks, and component inspections
3. What does the term 'clutch brake' refer to in a heavy-duty manual diesel drivetrain? → A device that stops the input shaft rotation to allow smooth gear engagement from neutral
4. What does a Variable Geometry Turbocharger (VGT) use to control boost pressure at different engine speeds? → Adjustable vanes in the turbine housing that redirect exhaust gas flow
5. How should a technician verify a brake systems repair is complete? → Perform a thorough road test, recheck all work, and verify with diagnostic tools
6. How should a technician verify a suspension and steering repair is complete? → Perform a thorough road test, recheck all work, and verify with diagnostic tools
7. What is the primary function of a differential in a diesel truck's drive axle? → To allow drive wheels to rotate at different speeds during turns
8. What does a sudden drop in oil pressure at operating temperature typically indicate in a diesel engine? → Worn main or rod bearings with excessive clearance allowing oil to escape
9. What tool is commonly used to measure voltage in a diesel electrical system? → Multimeter
10. How should a technician verify a engine diagnostics repair is complete? → Perform a thorough road test, recheck all work, and verify with diagnostic tools
11. What safety precaution is critical when performing heating and cooling work? → Following all manufacturer safety warnings, using proper PPE, and securing the vehicle
12. What diagnostic procedure is fundamental to electrical systems? → Systematic diagnostic process using scan tools, visual inspection, and test procedures
13. What is the function of a diesel fuel injector? → To spray fuel into the combustion chamber
14. What does a wiring diagram help a technician do? → Find electrical connections
15. What safety precaution is critical when performing safety inspections work? → Following all manufacturer safety warnings, using proper PPE, and securing the vehicle
16. What condition is indicated when a diesel truck's automatic transmission slips into neutral on its own during highway driving? → A failed output speed sensor or solenoid causing the TCM to default to a safe mode
17. What does 'cavitation' mean in the context of a diesel truck's hydraulic system? → Formation and collapse of vapor bubbles in the pump due to insufficient inlet pressure
18. Which air filter restriction measurement tool is most commonly used to determine when a diesel engine air filter requires replacement? → A manometer or restriction indicator on the air cleaner housing
19. What does a common rail fuel system do? → Maintains constant fuel pressure for injectors
20. What safety precaution is critical when performing emissions control work? → Following all manufacturer safety warnings, using proper PPE, and securing the vehicle
21. What should a diesel technician check when performing a coolant service to prevent liner cavitation erosion? → Supplemental coolant additive (SCA) concentration levels
22. What is the primary purpose of the charge air cooler (intercooler) in a turbocharged diesel engine? → To reduce the temperature of compressed air before it enters the intake manifold
23. Which component charges the battery in a diesel vehicle? → Alternator
24. What is the purpose of a hydraulic system return line filter on a diesel truck? → To remove contaminants from fluid returning to the reservoir before it is recirculated
25. A technician notices oil in the intake air piping between the turbocharger and the charge air cooler. What is the MOST likely cause? → Worn turbocharger compressor seal allowing oil to be ingested
26. What type of hydraulic seal material is most resistant to heat and petroleum-based hydraulic fluid degradation in diesel work truck applications? → Nitrile (Buna-N)
27. What is a symptom of air in a diesel fuel system? → Hard starting and rough idling
28. Which type of hydraulic pump is most commonly used in heavy-duty diesel truck-mounted hydraulic systems? → Gear pump
29. What is the most likely cause of blue-white smoke from a diesel engine that occurs primarily at idle and clears up at higher engine speeds? → Worn turbocharger seals allowing oil to be drawn into the intake under low-boost vacuum
30. Which component in the diesel lubrication system is responsible for removing solid contaminants from the oil before they reach engine bearings? → The oil filter