A9 Light-Vehicle Diesel Engines — practice test

High-yield A9 topics

The material that shows up year after year. If you're short on time, start here.

• High-pressure common rail, end to end. Lift pump feeds the HPFP, the inlet metering valve (MPROP/SCV) controls output, the rail sensor closes the loop, injectors dump excess to return. Rails run roughly 26,000-30,000 psi. Learn the split: rail won't build (supply, metering valve, pump) vs rail bleeds off (leaking injector, excessive return). Injector return/balance flow testing is fair game - the cylinder returning way more than its neighbors is your leaker, and it drags rail pressure down and kills hot restarts.
• CP4.2 failure and fuel contamination. On LML Duramax and 6.7 Power Stroke, a roller-follower failure sends metal through the whole fuel system. The tested point is repair scope: you do not just hang a new pump. Rail, injectors, lines, tank, and filter housing get replaced or flushed or the new pump dies too. Tie it to ULSD lubricity, ASTM D975, and the 15 ppm sulfur cap.
• Aftertreatment: DOC, DPF, SCR. Passive vs active regen; the DOC needs roughly 550-600F to light off and make the exotherm the DPF needs; active regen is post-injection or a dosing injector, not extra idle time. Soot burns, ash does not - a DPF still showing high delta-P after a successful forced regen is ash-loaded and gets cleaned or replaced, not regenned again. DEF is 32.5 percent urea, checked with a refractometer, and it freezing near 12F is by design.
• Cold-start systems, and they are not all glow plugs. Cummins light-duty uses an intake grid heater pulling serious current; Duramax and Power Stroke use glow plugs on a controller that varies on-time with coolant temp and can stay on after start. Know glow plug resistance checks (typically under about 2 ohms), amp-clamp testing the grid heater relays, and that a dead cold-start system gives you a rough cold start with white smoke that cleans up as it warms - it does not cause a warm no-start.
• Air side: VGT, EGR, and CCV. Sticky VGT vanes from soot give low boost and lazy power, or overboost if stuck closed, and often set actuator position-vs-desired codes. EGR cooler failures on 6.0/6.4 Power Stroke dump coolant into the intake - white sweet smoke, coolant loss, no external leak, pressure test the cooler. Crankcase pressure is a real diagnostic: high blowby with a plugged CCV pushes seals out. Do not chase a boost leak with a smoke machine before you check the CCV.
• Compression and cranking basics done the diesel way. Expect roughly 275-400 psi with no more than about 20 percent spread cylinder to cylinder, and know that cranking speed (around 150 rpm minimum) has to be right before the numbers mean anything. Diesels have no throttle plate, so there is no vacuum test and no fuel trim - your mechanical health data comes from compression, relative compression on a scope or scan tool, and cylinder contribution.

Where techs lose points on A9

Good technicians miss these — not from lack of skill, but because the question is built to catch them.

• Smoke color. On a gas engine white smoke means coolant. On a diesel white smoke is usually raw unburned fuel - low compression, bad glow plugs, low injection pressure, bad injector, or retarded timing. Coolant is a possible cause, but if the test gives you a cold-start white smoke that clears as it warms, the answer is the cold-start or combustion side, not a head gasket. Black is incomplete combustion (too much fuel or not enough air), blue is oil.
• Cylinder contribution/balance numbers are not misfire counters. The lowest-contributing cylinder does not automatically mean bad injector - low compression, a burned valve, or a leaking injector on a different cylinder pulling rail pressure all shift those numbers. The right next step is almost always compression or return-flow data, not an injector. Test writers love the 'just replace the injector' distractor.
• Low compression on ALL cylinders. The obvious pick is rings. The usual reality is slow cranking (weak batteries, high resistance in the cables, thick oil) or valve timing off a tooth. Verify cranking speed first - a low-but-even reading is a whole-engine cause, not eight sets of worn rings.
• Assuming every light diesel has glow plugs, or that a forced regen fixes a plugged DPF. Grid-heater trucks have no glow plugs to test. And a DPF that fails delta-P right after a completed regen is ash - regenning it again is the plausible-looking wrong answer.

A study plan that works for A9

Roughly two weeks of real preparation, in the order that actually builds on itself.

1. Days 1-4: Fuel system, both halves. Draw the low-pressure and high-pressure circuits from memory, including where each sensor sits. Drill rail-pressure logic (won't build vs bleeds off), injector return/balance testing, CP4 contamination scope, and ULSD/cetane/lubricity/gelling. This is the biggest single chunk on the test.
2. Days 5-7: Air induction, exhaust, and aftertreatment. VGT operation and stuck-vane symptoms, EGR and EGR cooler failures, CCV and crankcase pressure, then DOC/DPF/SCR: regen types, light-off temps, soot vs ash, DEF concentration and NOx sensors. Second-biggest chunk, and the area most techs skipped.
3. Days 8-10: General diagnosis plus the mechanical areas. Compression and relative compression procedure and specs, cranking speed, cold-start systems by platform, then head/valve train, block, oil, and cooling. Keep it to symptom-to-cause tables - the mechanical questions are shallower than the fuel ones.
4. Days 11-14: Practice tests only, in ASE format, including the two-technician items. Score, then go back and read the task list line for every miss. If you are under 80 percent on fuel or air/exhaust, do not touch anything else until you are - those two areas can pass or fail you by themselves.

Sample A9 questions

Straight from the bank — answers highlighted, with the explanation underneath.