To diagnose an engine noise, pin down when it happens: cold or hot, at idle or under load, in gear or in park. That single question narrows a knock or a tick to a few parts. It is also how the largest A1 area works: general engine diagnosis has 18 of the 45 scored questions, from 12 tasks, and its questions start from a symptom, such as a knock, a puff of smoke or a low reading, and ask what could cause it.
18scored questions on the test
40%of the scored questions
12tasks in the area
20questions on this page
What area A asks
From the 2026 task list, in our words:
Start from the complaint: verify it, road-test the vehicle, and check service history, bulletins and recalls.
No-crank, crank/no-start, hard start: decide whether the engine itself is at fault or another system is.
Leaks, noises and vibration: find the source, then decide on the repair.
Oil and coolant use, exhaust color and odor: work out where the oil or coolant is going.
Engine tests: manifold vacuum, power balance, cranking, relative and running compression, and leak-down, each read and acted on (reading engine test results).
PCV system: inspect and test its parts.
Tools: oscilloscope, DMM and scan tool, for mechanical and electrical faults alike.
Knock in drive, gone in park or neutral: something that only works under load, such as a cracked flexplate.
Tapping for a few seconds at the first start of the day: oil drained from the top of the engine overnight. A failed oil filter, whose anti-drainback valve no longer holds oil, is the sample answer.
Rattle at start-up that stops within seconds: a hydraulic timing chain tensioner waiting for oil pressure, or worn guides.
Ticking that follows engine speed: the valve train (lash, a lifter, a rocker) or an exhaust manifold leak, which ticks loudest cold and under acceleration.
Deep knock that grows with load: a rod bearing. Disable that cylinder and the knock changes.
Knock when cold that fades as the engine warms: piston slap.
Chirp that changes with rpm: a belt, a tensioner or pulley alignment, which belong to lubrication and cooling.
Smoke, oil use and coolant loss
Blue smoke is oil: worn rings, intake valve guides or stem seals, a clogged PCV system or turbocharger seals. Blue at start-up after sitting points to stem seals and guides; blue under load points to the rings.
White smoke with a sweet smell is coolant: a head gasket, a cracked head or block. Confirm with a cooling system pressure test and a combustion leak test.
Coolant loss with no leak in sight means it is going into the oil, the cylinders or the heater core. Check the oil and the cylinders before you condemn a gasket.
20 general diagnosis questions
0 of 20 answered · 0 correct
Question 1 of 20 · General Engine Diagnosis · 40%Not answered
During a cylinder leakage test on an engine with a rough idle, the gauge indicates 40% leakage. The technician hears a distinct hissing sound coming from the removed oil filler cap. Which of the following components is the most likely cause?
Explanation
Incorrect: A leaking exhaust valve or cracked manifold sends the air out of the tailpipe.
Correct: A large leak with the air hissing out of the oil filler means the air is getting past the piston into the crankcase, which points to worn rings.
Incorrect: A leaking intake valve sends the air back out of the intake and throttle body, not the oil filler.
Incorrect: A gasket leak into a coolant port shows up as bubbles in the radiator, not hissing at the filler.
Incorrect: The PCV system only vents the crankcase; it cannot create cylinder leakage.
Question 2 of 20 · General Engine Diagnosis · 40%Not answered
A vehicle exhibits a steady misfire on a single cylinder. The technician has confirmed spark and fuel delivery are functioning properly. Which sequence of mechanical engine testing is most appropriate to efficiently isolate the root cause?
Explanation
Incorrect: A cooling-system pressure test answers a coolant question that a steady misfire has not raised yet.
Incorrect: A vacuum reading is an overall engine check and does not confirm the problem is in this one cylinder.
Correct: A compression test quickly confirms the cylinder is mechanically weak, and a leak-down then shows where the air escapes: rings, valves or gasket.
Incorrect: A leak-down followed by a block test skips the quick compression check, and a block test only covers combustion gas in the coolant.
Incorrect: Exhaust testing does not locate a sealing fault inside one cylinder.
Question 3 of 20 · General Engine Diagnosis · 40%Not answered
During a cylinder leak-down test at Top Dead Center (TDC) on the compression stroke, a technician hears a distinct hissing sound coming directly from the vehicle's tailpipe. What does this specific diagnostic symptom indicate?
Explanation
Correct: Air hissing from the tailpipe means it is getting past a closed exhaust valve that is not sealing.
Incorrect: Worn rings or a worn bore leak air into the crankcase, which is heard at the oil filler.
Incorrect: A leaking intake valve sends air out of the intake and throttle body, not the tailpipe.
Incorrect: A cracked block casting leaks into the water jacket or crankcase, not out of the exhaust.
Incorrect: A blown head gasket sends air into a neighboring cylinder or the coolant.
Question 4 of 20 · General Engine Diagnosis · 40%Not answered
A technician measures compression on a single cylinder and records a slightly low reading. The vehicle's battery voltage dropped significantly during testing, resulting in a notably slow engine cranking speed. What is the most appropriate next step?
Explanation
Incorrect: A reading taken while the battery was sagging is artificially low and cannot be accepted as valid.
Incorrect: A wet test still uses the same weak battery and slow cranking, so its readings are no more trustworthy.
Incorrect: Pulling the head is a major teardown based on a test that was not done properly.
Incorrect: Replacing rings on the strength of a reading taken at low cranking speed condemns parts on bad data.
Correct: Slow cranking drags every compression reading down, so the battery has to be charged and the test repeated before the number means anything.
Question 5 of 20 · General Engine Diagnosis · 40%Not answered
A customer reports a loud, hollow knocking noise coming from the engine compartment when starting the vehicle in the morning. The technician notes that the noise noticeably decreases and almost entirely disappears as the engine reaches normal operating temperature. Which condition is the most likely cause?
Explanation
Incorrect: A sticking lifter makes a light tick at valve train speed, not a hollow knock.
Incorrect: Main bearing clearance gives a deep rumble that persists or worsens when hot.
Incorrect: A worn rod bearing knocks louder as the oil thins and the engine warms up, not quieter.
Incorrect: A cracked flexplate clunks or rattles at the back of the engine, and warming up does not quiet it.
Correct: A hollow knock that fades as the pistons expand to fill the bore is piston slap from excessive piston-to-wall clearance.
Question 6 of 20 · General Engine Diagnosis · 40%Not answered
A vehicle emits blue smoke from the exhaust during hard acceleration, but not at initial startup. The engine is naturally aspirated, and no smoke appears on deceleration. Which of the following is the most likely cause?
Explanation
Incorrect: A restricted air filter cuts airflow and power; it puts no oil into the cylinders.
Incorrect: A rich mixture makes black smoke, not blue.
Incorrect: A stuck-open PCV valve pulls the most vapor at high vacuum, at idle and on deceleration, not under hard acceleration.
Incorrect: Leaking valve stem seals smoke at startup or on deceleration, and neither happens here.
Correct: Blue smoke only under load, with none at startup or on deceleration, rules out the valve seals and guides and points to oil getting past worn rings.
Question 7 of 20 · General Engine Diagnosis · 40%Not answered
After a cylinder leak-down test on cylinder 6 shows 35% leakage with air escaping from the crankcase, which diagnostic step should be performed next before disassembly?
Explanation
Incorrect: Retorquing the head bolts treats a gasket problem the leak-down did not show, and torque-to-yield bolts cannot be retorqued.
Incorrect: Valve clearance affects valve sealing, and air into the crankcase is a ring or bore problem.
Incorrect: Pulling the head to look at valves ignores where the air escaped; crankcase leakage is not a valve problem.
Incorrect: Injectors have nothing to do with air leaking past the piston into the crankcase.
Correct: Crankcase leakage points at the rings and bore, and a borescope through the plug hole shows scoring or damage without taking anything apart.
Question 8 of 20 · General Engine Diagnosis · 40%Not answered
A cooling system pressure test holds steady at 15 psi for 10 minutes. However, a chemical block tester shows a positive reaction for combustion gases in the coolant. Which of the following is the best next step to confirm a head gasket breach?
Explanation
Incorrect: A thermostat controls coolant temperature and has nothing to do with combustion gas in the coolant.
Incorrect: Retightening head bolts without proof of the leak is a guess, and torque-to-yield bolts must not be retightened.
Correct: Pressurizing each cylinder with shop air and watching for bubbles at the radiator shows which cylinder is leaking into the coolant.
Incorrect: A compression test can show a weak cylinder but does not prove combustion gas is reaching the coolant.
Incorrect: The cap is not the issue: the system already held 15 psi, and a new cap cannot explain combustion gas in the coolant.
Question 9 of 20 · General Engine Diagnosis · 40%Not answered
After replacing a head gasket, a technician performs a leak-down test when the engine is cold; all cylinders are within specification. When repeated at operating temperature, cylinder 3 shows a 25% leakage increase and air escapes into the adjacent cylinder. What does this indicate about the repair?
Explanation
Correct: A leak that opens up only when the engine is hot is still a leak; the repair has failed because thermal expansion opens the path into the next cylinder.
Incorrect: Over-torqued bolts are a guess at the cause; the readings only show that the repair is leaking.
Incorrect: A head gasket does not need break-in time to seal; a leak into the next cylinder is a failure.
Incorrect: A leak-down test is valid on a warm engine, and repeating it hot is the right way to confirm the repair.
Incorrect: Passing a cold test does not clear the engine when the same cylinder leaks at operating temperature.
Question 10 of 20 · General Engine Diagnosis · 40%Not answered
A technician diagnoses and repairs a cracked intake manifold gasket that caused a P0171 (System Too Lean, Bank 1) code. What is the most appropriate final step to verify the repair and ensure quality control?
Explanation
Incorrect: Looking at the hoses again is not a measurement and does not confirm the trims have recovered.
Incorrect: The oxygen sensor reported the lean condition correctly, so replacing it adds cost and verifies nothing.
Incorrect: Clearing codes and releasing the car proves nothing about whether the lean condition is fixed.
Incorrect: Resetting the trims erases the evidence, and skipping the road test means the repair is never checked under load.
Correct: Fuel trims that return from their lean-correcting baseline to near zero prove the vacuum leak is gone.
Question 11 of 20 · General Engine Diagnosis · 40%Not answered
A technician performs a cylinder leak-down test. The tester is set to supply 100 psi, and the gauge reads 88 psi after the cylinder is pressurized. The manufacturer's maximum allowable leakage is 15%. Which of the following results is correct?
Explanation
Incorrect: 88 psi is the pressure the cylinder held, not the leakage; reading the gauge backwards gives the wrong percentage.
Correct: The cylinder lost 12 of the 100 psi supplied, which is 12% leakage, under the 15% limit, so it passes.
Incorrect: The 88 psi is the pressure held, not the leakage, so this answer reads the gauge backwards.
Incorrect: 12% is the right figure, but it is below the 15% maximum, so the cylinder does not fail.
Incorrect: Nothing in the readings gives 15%; the leakage is 100 minus 88, which is 12%.
Question 12 of 20 · General Engine Diagnosis · 40%Not answered
When performing a relative compression test using an amp clamp on the starter circuit, what would indicate a cylinder with a leaking valve?
Explanation
Correct: A leaking valve lowers the pressure the starter has to push against, so that cylinder's current peak is lower than the others.
Incorrect: A higher peak means the starter worked harder, which is a cylinder with more compression, not less.
Incorrect: The low peak lines up with one cylinder in the firing order; it is not a random pattern.
Incorrect: Even peaks across all cylinders are what a healthy engine shows, not one with a leaking valve.
Question 13 of 20 · General Engine Diagnosis · 40%Not answered
When performing a vacuum test at idle, what does a steady reading of 17-22 inches of mercury typically indicate?
Explanation
Incorrect: A blown head gasket gives a low or fluctuating reading as compression bleeds between cylinders.
Incorrect: A stuck PCV valve acts like a vacuum leak or restriction and moves the reading off normal.
Correct: A steady needle in the 17 to 22 inHg range at idle, near sea level, is what a healthy engine shows.
Incorrect: A burned exhaust valve makes the needle drop regularly each time that cylinder comes around.
Question 14 of 20 · General Engine Diagnosis · 40%Not answered
A technician hears a ticking from the valve train. Which tool best pinpoints the noisy component without disassembly?
Explanation
Incorrect: An oil pressure gauge can show a lubrication problem but cannot point to the ticking component.
Incorrect: A compression gauge measures cylinder sealing and gives no clue about where a tick comes from.
Correct: A stethoscope probe on each rocker area or lifter bore pinpoints exactly which component the tick is coming from.
Incorrect: A vacuum gauge shows overall engine condition, not which valve train part is noisy.
Question 15 of 20 · General Engine Diagnosis · 40%Not answered
During a cylinder balance test, a scan tool shows cylinder #4 RPM drop is only 2% while all other cylinders show 8-10% drops. Spark output and an injector balance test on cylinder #4 are both normal, and no misfire codes are stored. Which of the following should the technician do next?
Explanation
Correct: With spark and fuel already proven good, a cylinder that adds almost nothing has a mechanical problem, and a compression test is the next check.
Incorrect: Clearing codes and driving ignores a cylinder that is clearly not pulling its share.
Incorrect: The ECM's balance data matches a real weak cylinder; replacing the module treats the messenger as the fault.
Incorrect: The spark output test was normal, so a new coil replaces a part already shown to be good.
Incorrect: The injector balance test was normal, so a new injector replaces a part already shown to be good.
Question 16 of 20 · General Engine Diagnosis · 40%Not answered
What is a common symptom of a blown head gasket?
Explanation
Incorrect: Black smoke is unburned fuel from a rich mixture, not coolant.
Incorrect: A rough idle alone is not a distinguishing symptom; many faults cause it, while a gasket leak announces itself with white smoke.
Incorrect: Oil pressure depends on the pump and bearings; a head gasket leak does not normally lower it.
Correct: White smoke that persists after warm-up is coolant burning in the chamber, which is the classic head gasket symptom.
Question 17 of 20 · General Engine Diagnosis · 40%Not answered
An engine with mechanical (solid) lifters has a light, rhythmic ticking from under the valve cover that speeds up with engine rpm. What is the most likely cause?
Explanation
Incorrect: Injector ticking is normal, comes from the intake side, and is not a valve cover noise.
Correct: With solid lifters, too much clearance lets the rocker slap the valve tip on every opening, so the tick follows engine speed.
Incorrect: Worn rings cause blow-by and oil burning, not a rhythmic tick in the valve train.
Incorrect: A rod bearing makes a deeper knock from the bottom of the engine, not a light tick under the valve cover.
Question 18 of 20 · General Engine Diagnosis · 40%Not answered
A vehicle has normal oil consumption but excessive coolant loss with no visible leaks. The engine temperature remains normal. What is the MOST likely cause?
Explanation
Incorrect: A temperature sensor only reports temperature; it cannot make coolant disappear.
Correct: Coolant that disappears with no external trace is being burned or pushed out through an internal head gasket leak.
Incorrect: A faulty thermostat changes engine temperature, which stays normal here, and does not use up coolant.
Incorrect: A clogged radiator core makes the engine run hot, and the temperature here stays normal.
Question 19 of 20 · General Engine Diagnosis · 40%Not answered
When using a scan tool to diagnose a rough running engine, which parameter is MOST useful for identifying a misfiring cylinder?
Explanation
Incorrect: Fuel trims show a system-wide lean or rich correction, not a single cylinder.
Incorrect: Oxygen sensor voltage reacts to the whole bank's exhaust and cannot tell which cylinder misfired.
Correct: Misfire counters show the count for each cylinder, so they identify which cylinder is misfiring.
Incorrect: Coolant temperature has nothing to do with identifying which cylinder is misfiring.
Question 20 of 20 · General Engine Diagnosis · 40%Not answered
What does a wet compression test help determine?
Explanation
Incorrect: Cam lobe wear needs lift measurement or inspection, not oil squirted into a cylinder.
Incorrect: A wet test does not trace where coolant is leaking.
Incorrect: A compression gauge on a cylinder says nothing about how the oil pump is performing.
Correct: If oil raises a low reading, the rings were leaking; if it does not, the valves or gasket are the suspects.
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The exhaust guide trap
One official sample question has blue smoke on acceleration, with one technician blaming worn exhaust valve guides and the other worn exhaust valve seats. The marked answer is neither. Worn seats leak compression, not oil, and exhaust guides see little oil. Smoke on acceleration points to the rings; worn intake guides and seals smoke at idle and on deceleration, when manifold vacuum is high, and in a puff at start-up after oil has seeped past them overnight. Expect options that name a real oil path under the wrong driving condition.
Find where it is, then what changes it. A mechanic's stethoscope locates it; disabling cylinders one at a time separates a bearing knock, which changes when its cylinder stops firing, from valve-train noise; a short run with the drive belt off rules out the accessories.
What are the steps of engine diagnosis?
The A1 task list follows the shop sequence: verify the complaint and road-test, research service information and bulletins, decide which system is at fault, test and isolate the cause, then determine the repair. After the repair, verify the complaint is gone.
What does blue smoke from the exhaust mean?
The engine is burning oil. At start-up after sitting, suspect valve stem seals and intake guides; under load, suspect the rings. A clogged PCV system and turbocharger seals are other causes.
How many A1 questions are on general engine diagnosis?
18 of the 45 scored questions in the 2026 edition, the largest of the five areas.