Plenty of medium and heavy trucks on the road still run gasoline engines. Box trucks, school buses, and many Class 4 through 7 work trucks run big gas V8s. When one comes in with a misfire or a check engine light, somebody has to find the problem without throwing parts at it. The ASE T1 Gasoline Engines test is how you prove you’re that somebody.
I learned diagnosis the Army way: verify the complaint, test before you replace, and follow the evidence. That’s exactly what the T1 rewards. Here’s what’s on the test and a simple plan to get ready.
The T1 test at a glance
The T1 has about 50 scored questions, plus around 10 unscored research questions mixed in. You won’t know which ones don’t count, so treat every question like it does. You get about 75 minutes.
| Content area | Scored questions |
|---|---|
| A. General Engine Diagnosis | About 14 |
| B. Cylinder Head and Valve Train | About 4 |
| C. Engine Block | About 4 |
| D. Lubrication and Cooling Systems | About 3 |
| E. Ignition System | About 6 |
| F. Fuel, Air Induction, and Exhaust Systems | About 6 |
| G. Emissions Control Systems | About 5 |
| H. Computerized Engine Controls | About 8 |
| Total | About 50 |
To earn the certification, you also need two years of hands-on work experience. A diesel or heavy-equipment program can count for up to one of those years. Registration is $34 per order plus $62 per test.
A. General Engine Diagnosis (about 14 questions)
This is the biggest section. It’s about using basic tests to see what’s going on inside the engine.
Mechanical tests
- Complaint verification: Confirm the problem yourself and note when it happens. Use a scan tool and scope to back up what you hear and feel.
- Vacuum gauge: Steady but low means late timing. A regular drop means a burned or sticking valve. Vacuum falling at steady high rpm means a restricted exhaust.
- Compression and wet test: If adding oil raises a low reading a lot, it’s rings. If it barely changes, it’s valves or the head gasket. Two low cylinders side by side usually means a head gasket blown between them.
- Cylinder leakdown: Listen for where the air goes. Tailpipe is an exhaust valve, intake is an intake valve, oil fill is rings, and bubbles in the radiator are a head gasket or crack.
- Power balance: Kill one cylinder at a time and watch rpm. A cylinder that causes little or no drop isn’t pulling its weight.
Exhaust analysis and electrical
- Exhaust gas readings: High HC means unburned fuel, often a misfire. High CO with low O2 means rich. High O2 means lean or an exhaust leak ahead of the probe.
- Starting and charging: Voltage drop testing is the big one. A cable and its connections should only drop a few tenths of a volt while cranking. More than that means resistance to find.
B. Cylinder Head and Valve Train (about 4 questions)
Expect questions on inspection and measurement more than machining.
- Head warpage: Check with a straightedge and feeler gauge lengthwise, crosswise, and diagonally. Too much resurfacing raises compression and changes geometry.
- Valve guides and seals: Worn guides or seals cause blue smoke on startup and oil consumption.
- Springs, lifters, and timing: Check spring height, squareness, and pressure. Know how a jumped timing chain affects vacuum.
C. Engine Block (about 4 questions)
This section is about measuring and knowing which tool gives you which number.
- Cylinder bores: Taper is top of ring travel compared to the bottom. Out-of-round is two measurements 90 degrees apart at the same depth.
- Bearings and crankshaft: Plastigage checks oil clearance. A dial indicator checks crankshaft end play at the thrust bearing.
D. Lubrication and Cooling Systems (about 3 questions)
Only a few questions here, but they’re usually straightforward diagnosis.
- Oil pressure: Verify a low reading with a mechanical gauge. Low pressure at hot idle usually means worn bearings.
- Cooling system testing: Pressure test the system and the cap, check the thermostat, and use a block tester to find combustion gases in the coolant.
E. Ignition System (about 6 questions)
Most questions assume coil-on-plug ignition, but know waste spark basics too.
- Primary and secondary: On a scope, a high firing line means high resistance, like a wide gap or open wire. A low firing line means low resistance, like a fouled plug.
- Coils: Swap a coil to another cylinder. If the misfire follows the coil, it’s the coil. If it stays put, look at the plug, injector, or compression.
- Crank and cam sensors: No crank signal usually means no spark and no injector pulse. Check for rpm on the scan tool while cranking.
- Misfire codes: P0301 through P0308 point to one cylinder. P0300 is random or multiple misfire, so look for something common to all cylinders.
F. Fuel, Air Induction, and Exhaust Systems (about 6 questions)
Know the difference between pressure, volume, and holding pressure.
- Pumps, filters, and pressure: Good pressure with low volume means a restriction or weak pump. Pressure that bleeds down after shutoff points to a pump check valve, a leaking injector, or the regulator.
- Injectors: In a balance test, a small pressure drop means a restricted injector.
- Throttle bodies: Carbon buildup causes low or unstable idle. After cleaning or replacing an electronic throttle body, an idle relearn may be needed.
- Vacuum leaks and exhaust restriction: A smoke machine is the best way to find intake leaks. A plugged converter kills power under load.
G. Emissions Control Systems (about 5 questions)
Know what each system does and what happens when it fails open or closed.
- PCV: Plugged means crankcase pressure and oil leaks. Stuck open or a cracked hose means a lean condition and high idle.
- EGR: Stuck open causes rough idle or stalling. Not opening causes detonation under load and high NOx.
- Secondary air: Pumps air into the exhaust during warmup to help the converter light off.
- Catalytic converter: A good converter keeps the downstream O2 sensor fairly steady. If it switches like the upstream sensor, the converter is worn out.
- EVAP: Know purge and vent valve operation and smoke testing for leaks.
H. Computerized Engine Controls (about 8 questions)
This is the second biggest section. It’s about electrical testing and reading data, not memorizing codes.
- OBD II monitors: Misfire, fuel system, and comprehensive component monitors run continuously. Catalyst, EVAP, EGR, and O2 monitors run only when conditions are right. Clearing codes resets readiness.
- Fuel trim: Positive numbers mean the PCM is adding fuel for a lean condition. High positive trim at idle that goes away at higher rpm usually means a vacuum leak.
- Sensor and circuit testing: Check the 5-volt reference, signal, and ground. Sweep a TPS slowly and look for dropouts.
- Power and ground: Voltage drop on a PCM ground should be about 0.1 volt or less. Check powers and grounds before you condemn a module.
- Module replacement: A new PCM often has to be programmed with the correct calibration and VIN.
Try a few practice questions
Question 1
Vacuum reads normal at idle, but when the engine is held at 2,500 rpm the reading slowly drops toward zero. What is the most likely cause?
- A. A burned valve
- B. A restricted exhaust system
- C. Late ignition timing
- D. A leaking intake gasket
Answer: B. Exhaust backs up into the cylinders and vacuum falls off as rpm is held.
Question 2
A truck sets a code for the upstream oxygen sensor heater circuit. Which test should be done first?
- A. Replace the PCM
- B. Replace the catalytic converter
- C. Check the heater fuse, power feed, and heater element resistance
- D. Clear the code and road test
Answer: C. A heater circuit code is an electrical fault, so start with the fuse, power feed, and heater resistance.
Question 3
A secondary scope pattern shows a very low firing line on one cylinder. What is the most likely cause?
- A. A fouled spark plug
- B. An open plug wire
- C. A wide spark plug gap
- D. A lean mixture
Answer: A. A fouled plug gives the spark an easy path, so it takes less voltage to fire. The other three raise it.
A simple 4-week study plan
- Week 1, general engine diagnosis: Vacuum readings, compression, leakdown, power balance, exhaust gas, and voltage drop.
- Week 2, mechanical: Cylinder heads, valve train, block measurements, oil pressure, and cooling system testing.
- Week 3, ignition, fuel, and emissions: Scope patterns, coil testing, fuel pressure and volume, injectors, PCV, EGR, converters, and EVAP.
- Week 4, engine controls and review: Monitors, fuel trim, sensor circuits, and powers and grounds. Then take practice tests.
Keep going with the ASE T-Series
If you work on diesels too, check out the ASE T2 Diesel Engines study guide, or take a free practice test with instant explanations. T1 practice tests are coming soon.
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