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Automotive Service Excellence (ASE) A8 Engine Performance practice questions

230 free questions with answers and explanations.

Practice test
  1. 1.A technician is diagnosing a vehicle with an intermittent no-start condition. Using a scan tool, they observe that the engine RPM shows 0 during cranking, even though the engine is physically turning over. All other sensor readings appear normal. What sensor is most likely at fault?Computerized Engine Controls Diagnosis and Repair
  2. 2.A technician is diagnosing a vehicle with a complaint of intermittent rough running and occasional stalling. A scan tool reveals a stored DTC P0301 (Cylinder 1 Misfire Detected). The technician swaps the spark plug and ignition coil with cylinder 2, but the misfire remains on cylinder 1. Next, the technician measures the fuel injector resistance for cylinder 1 and finds it to be within specification. Which of the following should be the technician's next logical diagnostic step?Computerized Engine Controls Diagnosis and Repair
  3. 3.A technician is diagnosing a vehicle with a complaint of poor engine performance, particularly hesitation during acceleration, and a stored DTC P0171 (System Too Lean, Bank 1). Using a scan tool, the technician observes that Short Term Fuel Trims (STFT) are consistently positive (+20% to +25%) and Long Term Fuel Trims (LTFT) are also significantly positive (+15% to +20%) at idle and under light load. Which of the following is the MOST likely cause of these readings?Computerized Engine Controls Diagnosis and Repair
  4. 4.A technician is diagnosing a vehicle that exhibits a no-start condition, but cranks normally. There are no DTCs stored in the Powertrain Control Module (PCM). Using a scan tool, the technician observes that the engine RPM signal is present during cranking, but injector pulse width is 0 ms and ignition timing shows 0 degrees. Which of the following components should the technician investigate FIRST?Computerized Engine Controls Diagnosis and Repair
  5. 5.A technician is diagnosing a vehicle that started exhibiting a 'hard start' condition, requiring extended cranking, especially after sitting overnight. Once started, the engine runs normally, and there are no DTCs. The technician performs a fuel pressure test and observes that the fuel pressure drops rapidly from 50 PSI to 0 PSI within 5 minutes after the engine is shut off. What is the MOST likely cause of this symptom?Computerized Engine Controls Diagnosis and Repair
  6. 6.A technician is diagnosing a vehicle that exhibits an intermittent misfire on multiple cylinders, accompanied by a P0300 DTC (Random/Multiple Cylinder Misfire Detected). The scan tool shows that the engine's RPM sensor reading momentarily drops to zero or fluctuates erratically during the misfire events. Which sensor is MOST likely faulty?Computerized Engine Controls Diagnosis and Repair
  7. 7.A technician is diagnosing a vehicle with a complaint of a rough idle and occasional stalling, with no stored DTCs. Using a vacuum gauge, the technician observes a steady reading of 10 in.Hg at idle, where the specification is typically 17-21 in.Hg. The needle on the vacuum gauge shows a slight, rhythmic drop and recovery. What is the MOST likely cause of this symptom?Computerized Engine Controls Diagnosis and Repair
  8. 8.A client complains their vehicle has poor fuel economy and occasionally emits black smoke from the exhaust, especially during acceleration. A technician observes that the upstream oxygen (O2) sensor voltage remains consistently high (above 0.8V) during closed-loop operation. Which of the following is the MOST likely cause?Computerized Engine Controls Diagnosis and Repair
  9. 9.A vehicle owner reports that their vehicle occasionally stalls when coming to a stop, especially after extended highway driving. There are no DTCs stored. The technician observes that the engine RPM drops significantly below normal idle speed just before stalling. Which of the following is the most likely cause?Computerized Engine Controls Diagnosis and Repair
  10. 10.A technician is diagnosing a vehicle with a persistent Check Engine Light (CEL) and a P0420 diagnostic trouble code (DTC). The scan tool data shows that the Bank 1 Sensor 2 oxygen sensor voltage fluctuates rapidly, mirroring the Bank 1 Sensor 1 oxygen sensor. What is the most likely cause of this condition?Computerized Engine Controls Diagnosis and Repair
  11. 11.A technician is performing a diagnosis on a vehicle that has a P0442 DTC (Evaporative Emission System Leak Detected Small Leak). After performing a visual inspection and finding no obvious issues, the technician connects a smoke machine to the EVAP system. Where should the technician primarily focus their inspection for the smoke to locate the leak?Computerized Engine Controls Diagnosis and Repair
  12. 12.A technician is diagnosing a vehicle with a complaint of poor acceleration and a whistling noise from the engine compartment. A scan tool shows a high idle RPM (1200 RPM) and a Manifold Absolute Pressure (MAP) sensor reading of 80 kPa at idle. The manufacturer's specification for MAP at idle is typically around 30-40 kPa. Which of the following is the MOST likely cause of these readings?Computerized Engine Controls Diagnosis and Repair
  13. 13.A technician is performing a pre-purchase inspection on a vehicle. The Check Engine Light (CEL) is off, but the monitor status on the scan tool shows that the Evaporative Emission Control (EVAP) System monitor is 'Not Ready'. All other monitors are 'Ready'. What does this 'Not Ready' status indicate?Computerized Engine Controls Diagnosis and Repair
  14. 14.A client complains that their vehicle frequently stalls when the air conditioning is turned on, but otherwise runs fine. A technician observes that when the A/C is activated, the engine RPM drops significantly, and the engine struggles before stalling. A scan tool shows that the Idle Air Control (IAC) valve duty cycle remains largely unchanged (around 30-40%) even when the A/C is on. Which of the following is the MOST likely cause of this symptom?Computerized Engine Controls Diagnosis and Repair
  15. 15.A client complains their vehicle experiences intermittent stalling and rough idle, especially when coming to a stop. A scan tool shows no active DTCs, but during the rough idle condition, the technician observes the Manifold Absolute Pressure (MAP) sensor reading fluctuating rapidly between 18 in.Hg and 22 in.Hg at idle, where it should be relatively steady around 17-21 in.Hg. What is the MOST likely cause of this symptom?Computerized Engine Controls Diagnosis and Repair
  16. 16.A technician is performing a diagnosis on a vehicle with a complaint of a strong sulfur smell from the exhaust and a lack of power under acceleration. A scan tool shows a stored DTC P0420 (Catalyst System Efficiency Below Threshold, Bank 1). The upstream oxygen sensor (O2S11) switches rapidly, and the downstream oxygen sensor (O2S12) is also switching rapidly, mirroring the upstream sensor. What is the MOST likely cause of these symptoms?Computerized Engine Controls Diagnosis and Repair
  17. 17.A vehicle exhibits a rough idle, misfires under load, and a consistent P0301 DTC (Cylinder 1 Misfire Detected). The technician tests the ignition coil for cylinder 1 and finds it produces a strong spark. Next, the fuel injector for cylinder 1 is tested and found to be operating correctly. What is the most logical next step in diagnosing this misfire?Computerized Engine Controls Diagnosis and Repair
  18. 18.A vehicle owner reports a significant drop in fuel economy and a noticeable loss of power, especially under acceleration. The 'Check Engine' light is illuminated, and a DTC P0420 (Catalyst System Efficiency Below Threshold, Bank 1) is stored. During diagnosis, the technician measures exhaust back pressure before and after the catalytic converter. The reading before the converter is significantly higher than after. What does this indicate?Emissions Control Systems Diagnosis and Repair
  19. 19.A vehicle arrives at a shop with a complaint of a rough idle, especially when cold, and occasional misfires. There is a 'Check Engine' light illuminated, and DTC P0301 (Cylinder 1 Misfire Detected) is stored. The technician performs a cylinder balance test and confirms a misfire on cylinder 1. What is the LEAST likely cause of this specific misfire, considering the provided context?Emissions Control Systems Diagnosis and Repair
  20. 20.A vehicle with an Air Injection Reaction (AIR) system is diagnosed with a 'Check Engine' light and DTC P0410 (Secondary Air Injection System Malfunction). Inspection reveals that the AIR pump is running continuously, even when the engine is warm and at normal cruising speed. What is the MOST likely cause?Emissions Control Systems Diagnosis and Repair
  21. 21.A vehicle is being diagnosed for a 'Check Engine' light and a DTC P0455 (Evaporative Emission System Leak Detected, Gross Leak). After performing a smoke test, the technician observes smoke exiting from a crack in the charcoal canister. What is the MOST appropriate next step?Emissions Control Systems Diagnosis and Repair
  22. 22.A vehicle is brought in with a complaint of poor engine performance, especially at higher RPMs, and a 'Check Engine' light illuminated. Diagnostic trouble codes P0401 (EGR Flow Insufficient) and P0301 (Cylinder 1 Misfire) are present. After inspecting the EGR valve and finding it stuck open, what is the MOST likely cause of the P0301 misfire?Emissions Control Systems Diagnosis and Repair
  23. 23.A vehicle with an Air Injection Reaction (AIR) pump system has a 'Check Engine' light illuminated and DTC P0410 (Secondary Air Injection System Malfunction) stored. Upon inspection, the technician finds the AIR pump is running continuously, even after the engine has warmed up and during highway driving. What is the most likely cause?Emissions Control Systems Diagnosis and Repair
  24. 24.A vehicle fails its emissions test due to excessively high Nitrogen Oxides (NOx) levels, despite having a 'Check Engine' light off and no stored DTCs. The technician performs a road test and observes that the Exhaust Gas Recirculation (EGR) valve is not opening at all under conditions where it should (e.g., light-to-moderate acceleration). What is the MOST likely cause of the high NOx?Emissions Control Systems Diagnosis and Repair
  25. 25.A vehicle owner complains of a rotten egg smell from the exhaust, especially under hard acceleration. There are no 'Check Engine' lights illuminated, and no DTCs are stored. The vehicle's maintenance history indicates regular oil changes and tune-ups. What is the most likely cause of this odor?Emissions Control Systems Diagnosis and Repair
  26. 26.A technician is diagnosing a vehicle with a persistent P0101 (Mass Airflow (MAF) Sensor Performance) DTC. Live data shows the MAF sensor reading 0 g/s at idle, but increasing with RPM, though never reaching expected values. The engine runs rough at idle but smooths out somewhat at higher RPMs. What is the MOST likely cause?Emissions Control Systems Diagnosis and Repair
  27. 27.A vehicle owner complains that their vehicle occasionally stalls after refueling, especially if they 'top off' the tank. There are no 'Check Engine' lights illuminated. What is the MOST likely cause of this symptom?Emissions Control Systems Diagnosis and Repair
  28. 28.A technician is performing an emissions test on a vehicle. The test results show high levels of Hydrocarbons (HC) and Carbon Monoxide (CO), along with low Oxygen (O2) in the exhaust stream. The 'Check Engine' light is off, and there are no DTCs. What is the MOST likely cause of these readings?Emissions Control Systems Diagnosis and Repair
  29. 29.A vehicle owner complains of a noticeable decrease in power, especially under acceleration, and reduced fuel economy. The 'Check Engine' light is illuminated with DTC P0420 (Catalyst System Efficiency Below Threshold Bank 1). Upon inspection, the technician notices a strong sulfur smell from the exhaust. What is the most likely cause?Emissions Control Systems Diagnosis and Repair
  30. 30.A technician is diagnosing a vehicle with a 'Check Engine' light and a DTC P0440 (Evaporative Emission System Malfunction). The technician connects a scan tool and commands the EVAP purge valve to 0% duty cycle (closed) and then to 100% duty cycle (open). During the 100% duty cycle command, the fuel tank pressure (FTP) sensor reading changes significantly. What does this indicate about the purge valve?Emissions Control Systems Diagnosis and Repair
  31. 31.A vehicle is brought into the shop with a complaint of a strong fuel odor and difficulty starting after refueling. The 'Check Engine' light is on, and a scan tool retrieves DTC P0446 (Evaporative Emission Control System Vent Control Circuit Malfunction). Which of the following components is most likely at fault?Emissions Control Systems Diagnosis and Repair
  32. 32.A vehicle fails an emissions test due to high CO (Carbon Monoxide) and HC (Hydrocarbons) levels. The 'Check Engine' light is on, and DTC P0172 (System Too Rich Bank 1) is stored. The technician observes that the upstream oxygen sensor (O2S11) consistently reads around 0.9V, and the long-term fuel trim (LTFT) for Bank 1 is -20%. What is the most likely cause of this condition?Emissions Control Systems Diagnosis and Repair
  33. 33.A technician is diagnosing a vehicle that intermittently runs rough, especially during acceleration, and sometimes has a 'Check Engine' light with no specific DTCs. The vehicle uses a hot-wire Mass Air Flow (MAF) sensor. During a live data stream, the technician observes that the MAF sensor reading occasionally drops to 0.0 g/s for a brief moment before returning to normal, even though the engine is running. What is the most probable cause of this intermittent issue?Emissions Control Systems Diagnosis and Repair
  34. 34.A technician is diagnosing a vehicle with a fluctuating idle, especially after the engine warms up, and a 'Check Engine' light with DTC P0505 (Idle Control System Malfunction). The PCV valve was recently replaced. Inspection reveals that the PCV hose connecting to the intake manifold is collapsed and brittle. What is the MOST likely effect of this condition on engine operation?Emissions Control Systems Diagnosis and Repair
  35. 35.A vehicle fails an emissions test due to high NOx levels. The technician checks for DTCs, and none are found. A visual inspection shows no obvious vacuum leaks or exhaust restrictions. Using a scan tool, the technician observes that the Exhaust Gas Recirculation (EGR) valve is commanded open at cruising speeds, but the EGR position sensor indicates only 5% opening, while the specified value is 30%. What is the most likely cause of the high NOx emissions?Emissions Control Systems Diagnosis and Repair
  36. 36.A vehicle is being diagnosed for a 'Check Engine' light and DTC P0442 (Evaporative Emission System Leak Detected - Small Leak). The technician performs a smoke test on the EVAP system. Smoke is observed escaping from the area around the fuel filler neck, specifically from a cracked rubber hose connecting the filler neck to the fuel tank. What is the correct next step?Emissions Control Systems Diagnosis and Repair
  37. 37.A technician is diagnosing a vehicle with a 'Check Engine' light illuminated and DTC P0420 stored in the Engine Control Module (ECM). The freeze frame data shows the vehicle was operating at highway speeds with a warm engine. A scan tool live data stream reveals that the upstream oxygen sensor (O2S) fluctuates rapidly between 0.1V and 0.9V, while the downstream O2S shows a relatively flat reading hovering around 0.6V to 0.7V. What is the most likely cause of this condition?Emissions Control Systems Diagnosis and Repair
  38. 38.A technician is performing an emissions test on a vehicle. The test results show consistently low oxygen (O2) sensor voltage readings (close to 0.1V) at operating temperature and cruising speeds, even though the engine appears to be running smoothly. What does this indicate about the air-fuel mixture?Emissions Control Systems Diagnosis and Repair
  39. 39.A technician is diagnosing a vehicle with a rough idle, poor fuel economy, and a 'Check Engine' light illuminated. DTC P0171 (System Too Lean Bank 1) is stored. Live data shows a positive fuel trim of +20% at idle and +15% at highway speeds. A vacuum gauge connected to the engine shows a steady reading of 15 inHg at idle. What is the most likely cause of this condition?Emissions Control Systems Diagnosis and Repair
  40. 40.A vehicle experiences intermittent rough idle and occasional stalling, particularly when coming to a stop. There are no DTCs stored. The technician observes that the engine RPMs drop significantly when the brake pedal is depressed, and then recover. A visual inspection reveals a small, soft vacuum hose connected to the brake booster. What is the most likely cause of these symptoms?Emissions Control Systems Diagnosis and Repair
  41. 41.A technician is diagnosing a vehicle with a 'Check Engine' light and a DTC P0138 (O2 Sensor Circuit High Voltage, Bank 1 Sensor 2). Live data shows the downstream oxygen sensor (Sensor 2) voltage consistently at 0.9V. The upstream oxygen sensor (Sensor 1) is switching normally between 0.1V and 0.9V. What is the MOST likely cause of the P0138?Emissions Control Systems Diagnosis and Repair
  42. 42.A technician is diagnosing a vehicle that exhibits a rough idle, especially when cold, and occasional stalling. The 'Check Engine' light is illuminated, and a scan tool reveals a pending DTC P0404 (EGR Control Circuit Range/Performance). Which of the following is the most likely cause?Emissions Control Systems Diagnosis and Repair
  43. 43.A vehicle arrives with a strong smell of raw fuel from the exhaust, particularly after cold starts. The 'Check Engine' light is illuminated with multiple misfire codes (P0300, P0301, P0302). Additionally, the exhaust oxygen sensor voltage on Bank 1 is consistently reading high (around 0.9V). What is the MOST likely cause?Emissions Control Systems Diagnosis and Repair
  44. 44.A technician is diagnosing a vehicle with a P0133 (O2 Sensor Circuit Slow Response Bank 1 Sensor 1) DTC. The scan tool shows the upstream oxygen sensor (O2S11) switching slowly, taking over 500ms to go from rich to lean and vice versa, while the downstream oxygen sensor (O2S12) is stable at 0.7V. What is the most likely cause of this DTC?Emissions Control Systems Diagnosis and Repair
  45. 45.A technician is performing a pre-purchase inspection on a used vehicle. During the inspection, they notice that the Positive Crankcase Ventilation (PCV) valve rattles freely when shaken, and the hose connecting it to the intake manifold is soft and collapsing when the engine is running. Which of the following is the most appropriate action?Emissions Control Systems Diagnosis and Repair
  46. 46.A technician is diagnosing a vehicle with a 'Check Engine' light illuminated and a Diagnostic Trouble Code (DTC) P0171 (System Too Lean, Bank 1). The fuel trims are consistently positive at idle and under load. Which of the following is the MOST likely cause?Emissions Control Systems Diagnosis and Repair
  47. 47.A vehicle is brought into the shop with a strong fuel odor, especially after refueling. The 'Check Engine' light is on, and a DTC P0455 (Evaporative Emission System Leak Detected - Gross Leak/No Flow) is stored. The fuel tank pressure sensor shows a reading of 0.0 inches of H2O with the engine off and the fuel cap removed. What is the most likely initial step in diagnosing this issue?Emissions Control Systems Diagnosis and Repair
  48. 48.A vehicle arrives at the shop with a complaint of poor acceleration and black smoke from the exhaust. The technician connects a scan tool and observes that the mass air flow (MAF) sensor reading is significantly lower than expected for the engine's RPM and load. Which of the following is the MOST likely cause?Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
  49. 49.A technician is diagnosing a diesel vehicle equipped with a Selective Catalytic Reduction (SCR) system. The vehicle exhibits reduced engine power and a 'DEF System Malfunction' message on the dashboard. Upon inspection, the Diesel Exhaust Fluid (DEF) tank is full, and there are no obvious leaks. Which of the following is the MOST likely cause?Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
  50. 50.A technician is diagnosing a vehicle with a persistent P0171 (System Too Lean Bank 1) diagnostic trouble code (DTC). The fuel trims at idle are +20% short-term and +15% long-term. At 2500 RPM under load, the fuel trims drop to +5% short-term and +3% long-term. Which of the following is the MOST likely cause of this condition?Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair