# Overhaul Prep

> Independent exam prep for automotive, diesel and emergency-vehicle technicians: 12,000 practice questions across 81 certification tests, every answer explained in writing.

Overhaul Prep is built and run by a working technician. Questions are original, written to current task lists, and use the same formats as the real exams (direct, Technician A/B, EXCEPT, most-likely-cause). Every answer includes an explanation of *why* it is correct. Not affiliated with or endorsed by ASE, the EVT Certification Commission, the U.S. EPA, Cummins Inc., or Detroit Diesel.

Free to try with no credit card. Subscription: $16.99/month individual, $12.99/seat/month per seat for shops.

## Key pages

- [Free sample quiz](https://overhaulprep.app/sample): 10 real questions, no signup required.
- [All study guides](https://overhaulprep.app/articles): written guide for every test we cover.
- [Practice tests](https://overhaulprep.app/practice): index of all 81 certification tests.
- [Privacy](https://overhaulprep.app/privacy) · [Terms](https://overhaulprep.app/terms)

## Certification tests covered

### ASE Automobile
- [A1 — Engine Repair](https://overhaulprep.app/practice/A1): 119 practice questions. Half of A1 happens before you ever pull a head. That is where techs lose it.
- [A2 — Automatic Transmission / Transaxle](https://overhaulprep.app/practice/A2): 121 practice questions. High stall RPM means slipping clutches. Low stall means the stator. Miss that, miss A2.
- [A3 — Manual Drive Train & Axles](https://overhaulprep.app/practice/A3): 120 practice questions. Pinion depth sets the pattern, carrier shims set backlash. Mix those up and A3 ends you.
- [A4 — Suspension & Steering](https://overhaulprep.app/practice/A4): 120 practice questions. A4 fails techs who can measure a ball joint but cannot read a tire.
- [A5 — Brakes](https://overhaulprep.app/practice/A5): 122 practice questions. A5 fails techs who can do brakes but can't read a proportioning valve question.
- [A6 — Electrical / Electronic Systems](https://overhaulprep.app/practice/A6): 122 practice questions. Most A6 failures aren't theory problems. They're techs who never loaded the circuit.
- [A7 — Heating & Air Conditioning](https://overhaulprep.app/practice/A7): 121 practice questions. High head pressure is not always an overcharge. A7 asks that one five different ways.
- [A8 — Engine Performance](https://overhaulprep.app/practice/A8): 122 practice questions. A8 does not ask what a P0171 means. It asks what you do about it.
- [A9 — Light-Vehicle Diesel Engines](https://overhaulprep.app/practice/A9): 121 practice questions. Most A9 failures aren't diesel gaps. They're gas-engine habits applied to a diesel.

### ASE Medium / Heavy Truck
- [T1 — Gasoline Engines](https://overhaulprep.app/practice/T1): 140 practice questions. 17-21 in-Hg steady at idle. If you cannot read the needle, T1 will eat you alive.
- [T2 — Diesel Engines](https://overhaulprep.app/practice/T2): 151 practice questions. T2 fails techs who can pull an injector but cannot explain what the cutout test proved.
- [T3 — Drive Train](https://overhaulprep.app/practice/T3): 133 practice questions. If you cannot explain clutch brake squeeze without looking it up, T3 will find that hole.
- [T4 — Brakes](https://overhaulprep.app/practice/T4): 139 practice questions. If your fix for oil in the air tanks is a new dryer, T4 is going to eat you alive.
- [T5 — Suspension & Steering](https://overhaulprep.app/practice/T5): 131 practice questions. T5 fails techs on frame rails and dual matching, not kingpins.
- [T6 — Electrical / Electronic Systems](https://overhaulprep.app/practice/T6): 146 practice questions. Ohms on a dead cable prove nothing. T6 is won with voltage drops taken under load.
- [T7 — HVAC](https://overhaulprep.app/practice/T7): 135 practice questions. Both gauges reading low is not always low charge. That question sinks people on T7.
- [T8 — Preventive Maintenance Inspection (PMI)](https://overhaulprep.app/practice/T8): 136 practice questions. Most techs fail T8 on the numbers, not the wrenching: 4/32, 2 psi, 60 psi, 2 inches.
- [L2 — Electronic Diesel Engine Diagnosis](https://overhaulprep.app/practice/L2): 137 practice questions. L2 isn't a diesel test, it's a data test - and the composite booklet decides your score.

### EVT Fire Apparatus
- [DO1 — Driver/Operator Inspection](https://overhaulprep.app/practice/DO1): 86 practice questions. Most DO1 washouts aren't wrenching mistakes — they're operators who skip the air-brake check because 'it built up fine.'
- [F1 — Maintenance, Inspection & Testing](https://overhaulprep.app/practice/F1): 237 practice questions. Half the F1 room misses the vacuum test because they think 22 in. Hg has to hold.
- [F2 — Design & Performance Standards](https://overhaulprep.app/practice/F2): 238 practice questions. F2 is open book. Techs still fail it - because they cannot find the answer in time.
- [F3 — Fire Pumps & Accessories](https://overhaulprep.app/practice/F3): 238 practice questions. Most F3 failures aren't hydraulics. They're the transfer valve and the packing gland.
- [F4 — Fire Apparatus Electrical Systems](https://overhaulprep.app/practice/F4): 242 practice questions. The 11.8 V floor and a 120-second timer fail more apparatus than any bad alternator.
- [F5 — Aerial Fire Apparatus](https://overhaulprep.app/practice/F5): 237 practice questions. Most F5 misses are not hydraulics. They are the load chart with the waterway flowing.
- [F6 — Allison Automatic Transmissions](https://overhaulprep.app/practice/F6): 239 practice questions. First range and reverse both slip, everything else is fine. That's C5, not the pump.
- [F7 — Foam Systems](https://overhaulprep.app/practice/F7): 36 practice questions. Most F7 misses come from mixing up proportioning percentage with expansion ratio - they are not the same thing.
- [F8 — Hydraulic Systems](https://overhaulprep.app/practice/F8): 76 practice questions. F8 rewards the tech who remembers that a load that drifts down is a valve problem - not a pump problem.
- [FA4 — Advanced Electrical Systems](https://overhaulprep.app/practice/FA4): 36 practice questions. FA4 separates the parts-changers from the diagnosticians: multiple dead functions plus a data-link fault is a bus problem, not five coincidences.
- [GL — Ground Ladder Testing](https://overhaulprep.app/practice/GL): 36 practice questions. GL is a judgment test: a darkened heat-sensor label means remove-and-test, not replace-the-label.

### EVT Ambulance
- [E0 — Maintenance, Inspection & Testing](https://overhaulprep.app/practice/E0): 239 practice questions. E0 does not fail techs on exotic stuff. It fails them on 4/32, 9.6 volts, and 20 percent.
- [E1 — Ambulance Design & Performance](https://overhaulprep.app/practice/E1): 239 practice questions. KKK-A-1822 is dead. If you still quote Rev F numbers, E1 will bury you.
- [E2 — Ambulance Electrical Systems](https://overhaulprep.app/practice/E2): 243 practice questions. Most techs don't fail E2 on lightbars. They fail on shoreline, load shed, voltage drop.
- [E3 — Ambulance HVAC](https://overhaulprep.app/practice/E3): 241 practice questions. E3 fails techs who can diagnose a car A/C but have never touched a roof condenser.
- [E4 — Cab, Chassis & Powertrain](https://overhaulprep.app/practice/E4): 242 practice questions. E4 does not fail techs on engines. It fails them on idle, voltage, and weight.

### EVT ARFF (Airport)
- [AR1 — ARFF — Design & Performance (A-1)](https://overhaulprep.app/practice/AR1): 30 practice questions. AR1 starts with why an ARFF rig is built to sprint: response-time performance drives the whole design.
- [AR2 — ARFF — Chassis & Components (A-2)](https://overhaulprep.app/practice/AR2): 30 practice questions. AR2's theme: an ARFF rig ages by the calendar, not the odometer.
- [AR3 — ARFF — Extinguishing Agent Systems (A-3)](https://overhaulprep.app/practice/AR3): 30 practice questions. AR3 is about the agents: know which one goes on the fire, and how the truck actually throws it.

### ASE Advanced & EV
- [L1 — Advanced Engine Performance](https://overhaulprep.app/practice/L1): 163 practice questions. L1 does not ask what the code means. It asks why the code is there.
- [L3 — Light-Duty Hybrid / EV](https://overhaulprep.app/practice/L3): 157 practice questions. L3 is not an electrical test with a hybrid chapter. It is a power-flow test with electrical questions.
- [L4 — ADAS (Driver Assistance)](https://overhaulprep.app/practice/L4): 165 practice questions. The scan tool says "calibration complete." That does not mean the radar is aimed.
- [XEV — High-Voltage EV Safety](https://overhaulprep.app/practice/XEV): 162 practice questions. 400 V does not care that the dash says OFF. Verify zero energy or don't touch it.

### EPA A/C Certification
- [E609 — EPA MVAC A/C Refrigerant](https://overhaulprep.app/practice/E609): 161 practice questions. 609 is open book and guys still fail it. They answer from the shop, not from the reg.

### ASE Transit Bus
- [H1 — Transit Bus — CNG Engines](https://overhaulprep.app/practice/H1): 173 practice questions. CNG sits at 3600 psi. H1 cares more about the regulators than the engine.
- [H2 — Transit Bus — Diesel Engines](https://overhaulprep.app/practice/H2): 171 practice questions. A regen will never burn ash. That one fact is worth several questions on the H2.
- [H3 — Transit Bus — Drive Train](https://overhaulprep.app/practice/H3): 169 practice questions. H3 is not a truck test. Retarder logic and TCM inputs fail more techs than clutches.
- [H4 — Transit Bus — Brakes](https://overhaulprep.app/practice/H4): 172 practice questions. If your fix for a long pushrod is cranking the slack adjuster, H4 will fail you.
- [H5 — Transit Bus — Suspension & Steering](https://overhaulprep.app/practice/H5): 170 practice questions. A transit bus leaning at the curb is a leveling valve until you prove otherwise.
- [H6 — Transit Bus — Electrical / Electronic](https://overhaulprep.app/practice/H6): 157 practice questions. On a multiplexed bus the switch isn't a switch, it's an input. H6 punishes techs who forget it.
- [H7 — Transit Bus — HVAC](https://overhaulprep.app/practice/H7): 168 practice questions. H7 fails techs on the roof unit, not the dash: unloaders, reheat, and charging by weight.
- [H8 — Transit Bus — PM Inspection](https://overhaulprep.app/practice/H8): 173 practice questions. H8 is not a brake test. It is a paperwork test with brakes in it.

### ASE School Bus
- [S1 — School Bus — Body & Special Equipment](https://overhaulprep.app/practice/S1): 171 practice questions. Most S1 misses are not body work. They are the door switch, the transfer relay, and grounds.
- [S2 — School Bus — Diesel Engines](https://overhaulprep.app/practice/S2): 170 practice questions. Half the S2 low-power questions aren't the turbo. They're restriction you never measured.
- [S3 — School Bus — Drive Train](https://overhaulprep.app/practice/S3): 175 practice questions. The clutch brake is for launching from a dead stop, not for shifting. S3 will ask.
- [S4 — School Bus — Brakes](https://overhaulprep.app/practice/S4): 170 practice questions. S4 fails techs who crank an automatic slack adjuster to "fix" a long-stroke chamber.
- [S5 — School Bus — Suspension & Steering](https://overhaulprep.app/practice/S5): 170 practice questions. Half of S5 is steering, and half of that is knowing the gear is not what is loose.
- [S6 — School Bus — Electrical / Electronic](https://overhaulprep.app/practice/S6): 174 practice questions. Most S6 failures are not the 8-lamp system. They are voltage drop and bad grounds.
- [S7 — School Bus — HVAC](https://overhaulprep.app/practice/S7): 170 practice questions. Front vents blow cold, rear blows warm. S7 lives in the half of the bus behind you.

### ASE Truck Equipment
- [TE1 — Truck Equipment — Installation](https://overhaulprep.app/practice/TE1): 171 practice questions. Most TE1 failures are not electrical. They are a payload calculation nobody bothered to run.
- [TE2 — Truck Equipment — Electrical Install](https://overhaulprep.app/practice/TE2): 172 practice questions. TE2 fails techs who can splice but cannot calculate voltage drop on a 40-foot chassis run.
- [TE3 — Truck Equipment — Auxiliary Power](https://overhaulprep.app/practice/TE3): 168 practice questions. Flow makes speed. Pressure makes force. Half of TE3 is that one sentence.

### ASE Alternate Fuels
- [AF1 — Alternate Fuels — CNG](https://overhaulprep.app/practice/AF1): 171 practice questions. Methane rises. Propane sinks. Half of AF1 is checking whether you know which.

### ASE Collision Repair & Refinish
- [B2 — Collision — Painting & Refinishing](https://overhaulprep.app/practice/B2): 157 practice questions. B2 is not a spray test. Most guys fail it on mixing math and surface prep.
- [B3 — Collision — Non-Structural Repair](https://overhaulprep.app/practice/B3): 166 practice questions. On B3 the metal tells you the repair. Guess the steel grade and you fail the question.
- [B4 — Collision — Structural Repair](https://overhaulprep.app/practice/B4): 158 practice questions. If you cannot read a set of centering gauges, B4 ends you in the first 20 questions.
- [B5 — Collision — Mechanical & Electrical](https://overhaulprep.app/practice/B5): 159 practice questions. B5 fails techs who can panel-beat but cannot read a wiring diagram.
- [B6 — Collision — Damage Analysis & Estimating](https://overhaulprep.app/practice/B6): 163 practice questions. Most B6 failures aren't damage analysis. They're the P-pages nobody reads.

### ASE Maintenance & Light Repair
- [G1 — Auto Maintenance & Light Repair](https://overhaulprep.app/practice/G1): 160 practice questions. Half of G1 is brakes and steering. The guys who fail spent all week on engines.

### Cummins
- [CX15 — X15 / ISX15 Heavy-Duty](https://overhaulprep.app/practice/CX15): 120 practice questions. FC 415 with 50 psi on a mechanical gauge is a sensor fault, not a bearing fault.
- [CX12 — ISX12 / X12](https://overhaulprep.app/practice/CX12): 120 practice questions. The X12 will fail you on a shared 5-volt short, not on how well you know the XPI pump.
- [CMR — B6.7 & L9 Mid-Range](https://overhaulprep.app/practice/CMR): 119 practice questions. Mixing up B6.7 and L9 valve lash sinks more techs on CMR than any hard question.
- [CFUEL — XPI Common-Rail Fuel](https://overhaulprep.app/practice/CFUEL): 115 practice questions. Unplug the fueling actuator and rail pressure climbs. That pump isn't your problem.
- [CATS — Aftertreatment & Emissions](https://overhaulprep.app/practice/CATS): 119 practice questions. A refractometer reading of 32.5% does not prove the DEF is good. That one fails techs.
- [CELEC — Electrical & Sensors](https://overhaulprep.app/practice/CELEC): 120 practice questions. 60 ohms across CAN High and Low is a pass. 120 ohms is half a network.
- [CDIAG — INSITE & Controls](https://overhaulprep.app/practice/CDIAG): 120 practice questions. INSITE told you the injector is bad. INSITE is wrong. Prove it before you cut the seal.

### Detroit Diesel
- [DD15 — DD13 / DD15 Heavy-Duty](https://overhaulprep.app/practice/DD15): 119 practice questions. Asymmetric turbo, no vanes. Rail pressure is not injection pressure. Start there.
- [DD16 — DD16 Heavy-Duty](https://overhaulprep.app/practice/DD16): 120 practice questions. The DD16 rail lives near 900 bar. The injector amplifies the rest. Miss that, fail the test.
- [DFUEL — Amplified Common-Rail Fuel](https://overhaulprep.app/practice/DFUEL): 119 practice questions. 900 bar in the rail is normal on ACRS. Condemn the pump over it and you fail DFUEL.
- [DATS — Aftertreatment / One-Box](https://overhaulprep.app/practice/DATS): 118 practice questions. A 4364 on a One-Box is almost never the outlet NOx sensor. That's the whole test.
- [DDEC — DDEC Controls & DiagnosticLink](https://overhaulprep.app/practice/DDEC): 118 practice questions. The MCM is fuel-cooled. Miss that on the test and you miss it in the bay too.
- [DAXLE — Detroit Axles](https://overhaulprep.app/practice/DAXLE): 120 practice questions. DAXLE fails techs on one line: heavy truck wheel bearings finish with end play, not preload.
- [DT12 — DT12 Automated Manual](https://overhaulprep.app/practice/DT12): 116 practice questions. Half the DT12 "no-shift" tickets I have seen were a 60 psi air gauge, not a bad TCU.