ASE A1 common mistakes
ASE A1 · Understand the traps and how to avoid them.
Condemning a major component (catalytic converter, head gasket, oil pump) off a single stored DTC or a scan-tool 'likely cause' list, then replacing it before running the pinpoint test for that code.
Candidates treat diagnostic trouble codes as component verdicts rather than as descriptions of a symptom the PCM observed, and they are under time pressure to name a repair.
What to do instead
For every code, write down the test that proves or disproves the part before touching a wrench: a backpressure/vacuum reading for a P0420, a block-test and pressure test for a P0301 with coolant loss, a mechanical gauge for oil pressure. Do not authorize a part until you can state the measured value that justifies it.
Running a compression test with the throttle closed, fewer than five compression strokes, and no wet retest, then calling a low cylinder 'bad rings'.
The procedure feels mechanical and obvious, so candidates skip the standardized steps and stop at the first low number rather than differentiating where the pressure is escaping.
What to do instead
Hold the throttle wide open, disable ignition and fuel, crank an equal number of strokes on every cylinder, record all readings, then add a few drops of oil and repeat any low cylinder. A wet reading that comes up points at rings/bore; one that stays flat points at valves, head gasket, or a cam timing problem. Confirm with a leak-down test before disassembly.
Bolting a resurfaced or replaced cylinder head back on without checking valve stem-to-guide clearance, installed spring height, and the resulting pushrod length or valve lash.
Candidates focus on the head gasket and torque sequence and treat valvetrain geometry as 'it came apart, so it goes back together the same way', forgetting that milling changes the distance from cam to crank.
What to do instead
Before assembly, measure guide clearance with a dial indicator or bore gauge, measure spring installed height against spec, and on adjustable or non-adjustable valvetrains recheck pushrod length or lash after any deck or seat work. Record the numbers on the repair order.
Setting cam timing to the marks, then never verifying with a second check - no #1 TDC confirmation on the compression stroke, no phaser-to-cam dowel verification, no rotation by hand before cranking.
Timing marks look unambiguous, and candidates rotate the engine with the starter first instead of by hand, so an off-a-tooth or misindexed VVT phaser shows up as bent valves rather than as a caught mistake.
What to do instead
Confirm #1 is on TDC of compression (both valves closed, rotor/trigger aligned), index the phaser to its dowel or pin, then turn the engine two full revolutions by hand and re-verify every mark before the first key crank. On interference engines, this step is mandatory, not optional.
Measuring cylinder bores with a telescope gauge and micrometer only at the top of the bore, and without rocking the gauge to find the true maximum diameter.
Taper and out-of-round are invisible to the eye, and candidates take one convenient measurement near the ridge where the bore is smallest rather than where the wear actually is.
What to do instead
Measure each bore at three heights (near the deck, mid-stroke, below the ring travel) in both the thrust and axial directions, rocking the telescope gauge to seat it on the largest diameter, then compare to the service spec for taper and out-of-round. Re-measure after any hone.
Diagnosing an overheating complaint by replacing the thermostat, then filling the cooling system and returning the car without pressure testing or bleeding air from the system.
Thermostats are cheap and overheating 'is usually the thermostat', and trapped air or a seeping head gasket reproduce the same gauge reading, so the comeback looks like a bad part rather than a missed test.
What to do instead
Pressure test the system cold and hot, check for combustion gases in the coolant, verify fan operation and actual outlet temperature with an infrared thermometer, and follow the manufacturer's bleed/fill procedure (heater on, bleed screw or vacuum fill) before releasing the vehicle.
Replacing an oil pump or sending unit based on the dash gauge or a low oil pressure light, without verifying with a mechanical gauge plumbed into the gallery.
The dash gauge is the only pressure information the candidate looks at, and the sender is easy to blame because it is easy to reach.
What to do instead
Tee in a known-good mechanical gauge at the sender port, compare hot idle and 2000 rpm readings to spec, and note viscosity and filter condition before condemning anything. Only after the mechanical reading is confirmed low do you move to pickup, clearances, relief valve, or pump.
Chasing a misfire by loading the parts cannon - coils, plugs, wires - using a spark check that just lays a plug against the block.
A spark that jumps in open air at atmospheric pressure proves almost nothing about the coil's ability to fire a lean, high-cylinder-pressure mixture, and the misfire may be fuel or mechanical rather than ignition.
What to do instead
Use an adjustable spark tester set to a real gap, then isolate whether the cylinder is ignition, fuel, or mechanical: swap the coil/injector to a neighbor and watch whether the misfire follows, check injector pulse and fuel trim, and run a relative compression or power-balance test before replacing anything.
Diagnosing a no-start or no-charge by swapping the starter or alternator after a parts-store bench test, skipping voltage-drop tests on the cables and grounds.
The bench test 'passed' or 'failed' and feels authoritative, but high resistance in a cable or ground produces the identical symptom and is invisible to a bench test.
What to do instead
Measure cranking voltage at the battery and at the starter, and perform a voltage drop on the positive and ground circuits under load (typically under 0.5 V per leg). For charging, check field control and voltage drop on the output circuit before removing the alternator.