ASE4 Cheat Sheet 2026

The 30 highest-yield ASE4 facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. A spark plug gap measures 0.065 inches; specification is 0.040 inches. The effect on ignition will be: → Higher voltage demand that may cause misfires
2. An engine equipped with port fuel injection has a rough idle that improves when the air filter is removed. This MOST likely indicates: → A severely restricted air filter causing a lean condition
3. What is the purpose of the ECM's adaptive fuel strategy (long-term fuel trim)? → To learn and compensate for long-term changes in engine components
4. A technician notices LTFT is -15% on both banks. This indicates the ECM is removing fuel because the engine is: → Running rich and subtracting fuel to maintain stoichiometry
5. A cold engine starts and idles but stalls when the throttle is opened. Fuel pressure is normal. The MOST likely cause is: → A dirty or failed mass airflow sensor
6. A scan tool shows the ECT (engine coolant temperature) PID reads 195°F with the engine warm. The cooling fan is cycling normally. This reading indicates: → Normal operating temperature
7. A technician observes crossfire between two adjacent ignition wires. The MOST likely cause is: → Wires routed too closely together or damaged insulation
8. A steady vacuum reading of 17–21 in-Hg at idle is considered: → Normal for a healthy engine at sea level
9. Which OBD II readiness monitor would most directly be affected by a faulty EVAP purge solenoid valve? → Evaporative emission (EVAP) monitor
10. A vehicle has high NOx readings on an emissions test but passes HC and CO. The MOST likely cause is: → A faulty or clogged EGR system
11. A DTC with the prefix 'P0' indicates what type of fault code? → Generic (SAE-standardized) powertrain fault
12. A clogged PCV valve can cause which of the following symptoms? → Oil leaks due to excessive crankcase pressure
13. A short-term fuel trim (STFT) reading of +15% indicates the ECM is: → Adding fuel to compensate for a lean condition detected by the O2 sensor
14. Which sensor is MOST responsible for closed-loop fuel control correction? → Upstream oxygen (O2) sensor
15. When Mode $04 is executed from a scan tool, what action does it perform? → Clears all stored DTCs, freeze frame data, and resets readiness monitors to incomplete
16. A knock sensor DTC is set and the engine has excessive spark retard. The knock sensor circuit tests good. The NEXT step is to: → Check for carbon deposits causing actual detonation
17. A stuck-open EGR valve at idle will cause: → Rough idle and possible stalling
18. An intermittent misfire code P0301 is stored but no misfire occurs at idle. The BEST next step is: → Road-test the vehicle under the conditions that set the code
19. A technician performs a fuel injector balance test. One injector causes a smaller pressure drop than the others. This means that injector is: → Flowing too little fuel (clogged or weak)
20. During a cylinder balance test, disabling cylinder #5 drops RPM by 50 while all others drop by 200. This indicates: → #5 is weak and contributing less power
21. Which condition will cause the ECM to enter open-loop mode even with a warm engine? → Hard acceleration (WOT enrichment)
22. A vacuum gauge needle fluctuates between 14 and 19 in-Hg at idle. This suggests: → An intermittent misfire or sticking valve
23. A no-start complaint: the engine cranks normally but will not start. Battery and charging tests are normal. The technician should next check: → Spark, fuel pressure, and injector pulse with a noid light
24. A technician finds fuel pressure holds for 30 seconds after the engine is shut off but then bleeds down slowly over several minutes. This MOST likely indicates: → A leaking fuel injector
25. What is the purpose of the secondary ignition system's high voltage? → To ionize the spark plug gap and initiate combustion
26. During an ignition coil primary voltage test with a lab scope, the technician sees no voltage drop across the coil. This indicates: → The coil driver transistor or ECM output is not switching
27. When a scan tool shows LTFT of +22% at idle that drops to +3% at 2500 RPM, the most likely diagnosis is: → A vacuum leak causing an idle-specific lean condition
28. What does MIL stand for in OBD II diagnostics? → Malfunction Indicator Lamp
29. Low compression on two adjacent cylinders most likely indicates: → A blown head gasket between those cylinders
30. When performing a cylinder leakage test, air is heard escaping from the throttle body. This indicates: → A leaking intake valve