There is no single ASE diesel certification. A diesel mechanic is certified on the test that matches the vehicle: T2 Diesel Engines for medium and heavy trucks, A9 Light Vehicle Diesel Engines for pickups and vans, H2 for transit buses and S2 for school buses. L2 sits above them as an advanced electronic diagnosis test. Here is how they differ and which one to take first.
The diesel tests side by side
Checked Oct 23, 2026 against each source linked in the table. The T-Series row covers all eight truck tests; T2's own figures are in the text below. H2 and S2 are not on this site yet.
T2 Diesel Engines is the heavy-duty test: 65 questions (55 scored + 10 unscored), 90 minutes, the largest of the truck tests. It is one of the seven Master truck tests, and one of the four truck tests with an official ASE practice test (T2, T4, T6, T8 only).
A9 Light Vehicle Diesel Engines covers diesel pickups, vans and cars: 50 scored questions in 75 min. It sits in the A series, so the Renewal App covers it, but it is not required for Master.
L2 Electronic Diesel Engine Diagnosis Specialist is the advanced step. You must have passed one diesel engine test (A9, H2, S2 or T2) and one electrical test (A6, H6, S6 or T6) first, and it needs 3 years of experience. Its fee is $124 against $62 for T2 or A9.
H2 and S2 are named with A9 and T2 as diesel engine tests in the L2 prerequisite; they belong to the Transit Bus and School Bus series, which this site doesn't cover yet.
Work on medium and heavy trucks? Start with T2. Mostly diesel pickups and vans? Start with A9.
Becoming an ASE certified diesel mechanic
Get the experience:2 years of hands-on work, less with approved training (see requirements).
Pass the test: first attempts are test center only; you can test before the experience is approved.
Build toward Master truck: T2 plus T3–T8. T2–T8 make Master Medium-Heavy Truck Technician; T1 is not required. All seven in one order cost $468 (as of Oct 2026).
Keep it current:5 years, then a recertification test. The Renewal App is not for T tests: T2 is renewed by a recertification test (28 questions, 45 minutes).
Per the Bureau of Labor Statistics (page updated Aug 27, 2026), diesel service technicians earned a median $61,770 in 2025, the typical entry is a high school diploma plus long-term on-the-job training, and some need a commercial driver's license to road-test trucks and buses.
Diesel “certification online”
Online diesel courses teach; they don't certify. A first attempt at T2, A9 or L2 is test center only; only recertification can be taken with remote proctoring at home.
A truck battery is found frozen after sitting discharged in freezing weather. The technician should:
Explanation
Incorrect: Fast charging frozen electrolyte can rupture or explode the battery.
Incorrect: A load test on a frozen battery is unsafe and gives a meaningless reading.
Incorrect: Jump-starting through a frozen battery puts heavy current through it and risks an explosion.
Correct: A frozen battery is let to thaw before charging is even tried, because charging or loading frozen electrolyte can make the case crack or the battery explode; it is then checked for damage.
An engine stays at low idle when the accelerator pedal is pressed. The scan tool shows pedal position changing, and an idle validation switch (IVS) fault code is active. Which of these could be the cause?
Explanation
Incorrect: An EGR valve stuck open causes rough running and smoke, not an engine locked at low idle with an IVS code.
Correct: The engine module checks pedal position against the idle validation switch; when they disagree, it holds the engine at low idle, which matches the IVS code here.
Incorrect: A restricted secondary fuel filter costs power under load, but the engine would still respond to the pedal and no IVS code would be set.
Incorrect: A failed intake manifold pressure sensor affects fueling and boost readings, not pedal validation.
T2 Diesel Engines for medium and heavy trucks, A9 for light-vehicle diesels. L2 is the advanced diagnosis test that comes after one of them plus an electrical test.