Cert IV Auto Diagnostic cheat sheet

Diagnostic order: verify the complaint, confirm the symptom, then test before replacing parts; never fit components on assumption.

DTC scan is a starting point, not a diagnosis: freeze-frame data, pending vs current status and readiness monitors must be interpreted together.

Fuel trim rule: positive long-term trim above about +10% indicates a lean condition (vacuum leak, low fuel pressure); negative trim indicates rich.

Voltage drop testing: measure across a loaded circuit, not to ground; more than 0.5 V drop on a power or ground path is excessive.

Compression test: cranking with throttle open and all plugs removed; cylinders should be within 10% of each other and above manufacturer minimum.

Cylinder leakage test: air applied at TDC on compression stroke; leakage past intake = intake valve, past exhaust = exhaust valve, into crankcase = rings.

Oscilloscope patterns: ignition secondary burn time typically 1.5-2.5 ms; excessive burn time suggests high resistance in the secondary circuit.

Five-gas analysis: high CO = rich mixture, high O2 and low CO2 = lean or misfire, high HC = incomplete combustion or misfire.

Road test with live data before and after repair; a cleared DTC requires a drive cycle to confirm the fault is gone, not just hidden.

No-start checks in order: battery state of charge, cranking speed, spark or injection pulse, fuel pressure, then mechanical (compression, timing).

Fuel pressure specification is measured with the pump running and regulator reference connected; leak-down rate indicates a failing check valve or injector.

Vacuum gauge reading at idle of 17-22 inHg steady is normal; fluctuating needle indicates valve or ignition fault, low steady reading indicates leak or late timing.

Timing belt or chain faults show as cam/crank correlation codes; verify mechanical timing marks before replacing sensors.

Coolant loss with no external leak and white exhaust suggests head gasket or cracked head; confirm with a combustion gas test in the cooling system.

Oil consumption and blue smoke on acceleration point to worn valve guides or seals; on deceleration, worn rings are more likely.

Engine mount failure shows as clunk under load changes; check by loading the engine in drive and reverse with the brake applied.

Driveline vibration diagnosis: vibration speed-related and load-sensitive suggests CV joint or driveshaft; engine-speed-related suggests flexplate or converter.

Ohm's law: V = I x R; power P = V x I. Use these to size fuses and predict circuit behaviour.

Series circuit: current is common, voltage divides; parallel circuit: voltage is common, current divides.

Battery load test: at 12.6 V open circuit the battery is at rest and charged; below 12.4 V recharge before testing; cranking should not fall below about 9.6 V.

Parasitic draw: measure with a clamp meter or in series after modules sleep; more than about 50 mA after sleep is excessive.

Charging: 13.8-14.8 V at idle with loads off is typical; low output at idle but normal at raised rpm suggests belt slip or faulty alternator.

CAN bus diagnosis: measure resistance between CAN high and CAN low with power off; about 60 ohms indicates two 120-ohm terminators present.

Sensor testing: three-wire sensors use reference, signal and ground; a signal stuck at reference or ground voltage suggests open or shorted wiring.

Relay test: confirm control coil resistance, then verify switched output under load; a relay that clicks does not prove contact integrity.

Brake pedal pulsation under light braking usually indicates disc thickness variation or lateral runout; measure runout with a dial gauge.

Brake fluid is hygroscopic; moisture above about 3% by volume lowers boiling point and warrants replacement.

Pulling under braking indicates a seized caliper piston or collapsed hose; pulling while driving indicates alignment or tyre pressure.

Tyre diagnosis: wear on both edges = underinflation; centre wear = overinflation; one-sided wear = camber or toe fault.

Wheel alignment: toe-in or toe-out causes feathered tread; camber causes one-sided wear; caster affects straight-line stability and return to centre.

Steering play check: with wheels straight ahead, free play at the rim should not exceed manufacturer specification, typically 20-30 mm.

Shock absorber test: bounce the corner and release; more than one rebound oscillation indicates worn damping.

ABS fault diagnosis: wheel speed sensor air gap and tone ring condition are the most common causes of intermittent ABS warning lamps.

Automatic transmission fluid level is checked with the engine running and fluid at operating temperature, in park or neutral per manufacturer procedure.

Fluid condition: dark or burnt fluid with a varnish smell indicates overheating; milky fluid indicates coolant contamination via the cooler.

Slipping under load with correct fluid level suggests low line pressure, worn clutches or a faulty pressure control solenoid.

Harsh or delayed shifts often trace to a sticking shift solenoid, valve body contamination or incorrect adaptive learning values.

Torque converter clutch shudder appears as vibration at light throttle around lock-up speed; confirm with live data on TCC slip.

CV joint diagnosis: clicking on tight turns indicates outer joint wear; vibration under acceleration indicates inner joint or shaft imbalance.

Manual gearbox: difficult engagement with the engine running but easy with it off suggests clutch drag or a hydraulic fault.

Differential noise: whine that changes with speed but not gear selection points to final drive bearings or gear set wear.

Refrigerant pressure relationship: at 20 degrees C ambient, a static R134a system reads about 470 kPa; R1234yf reads lower and requires dedicated equipment.

High side pressure too high with normal low side suggests condenser airflow restriction, overcharge or non-condensable gas.

Low side pressure too high with poor cooling suggests a worn compressor or faulty expansion valve.

Both sides low with bubbles in the sight glass suggests undercharge; leak test with an electronic detector or UV dye before recharging.

Refrigerant must never be vented to atmosphere; recovery and recycling equipment is mandatory.

Cabin filter restriction reduces airflow; measure blower current and vent airflow before condemning the blower motor.

Blend door actuator faults show as temperature stuck on one side; confirm with actuator sweep and door position feedback.

Compressor clutch engagement requires adequate refrigerant pressure; a low-pressure cutout prevents clutch engagement when undercharged.

Before lifting a vehicle, confirm jack and stand capacity exceeds the vehicle's kerb mass and lift only at manufacturer lift points.

Support a lifted vehicle on rated stands; never work under a vehicle supported only by a jack.

Disconnect the battery negative terminal before working on airbag, SRS or fuel systems; wait the manufacturer-specified time for capacitor discharge.

Hybrid and electric vehicle high-voltage systems are typically above 60 V DC; only trained and authorised personnel may isolate and verify de-energisation.

Brake and clutch dust may contain asbestos in older vehicles; use wet methods and approved respiratory protection, never compressed air.

Fuel system work requires an extinguisher within reach, no ignition sources, and a well-ventilated area.

Personal protective equipment: safety glasses for under-vehicle work, gloves for chemicals, hearing protection for impact tools above 85 dB.

Lockout/tagout applies to workshop equipment and vehicle systems before maintenance; verify zero energy state before repair.

Waste streams are segregated: used oil, coolant, refrigerant, batteries, tyres and solvent each require separate containment and licensed disposal.

Used oil must never enter drains or soil; store in labelled, lidded containers and record volumes transferred.

Spill kits must be stocked and accessible; absorbent material contaminated with oil is itself a regulated waste.

Refrigerant recovery efficiency is regulated; recovered refrigerant is weighed and logged, and non-condensable gas is separated.

Parts washing solvents require closed units and scheduled replacement; sludge is hazardous waste.

Housekeeping rule: clear walkways, no oil on floors, and containers closed when not in use reduce both slip and fire risk.

Battery storage: lead-acid batteries are stored upright on impermeable bunding and recycled through a licensed carrier.

Air and water discharges from the workshop must not exceed site permit limits; compressor condensate is treated as trade waste.

A job card must record the complaint in the customer's words, the diagnosis performed, parts fitted and the final road test result.

Repair order authorisation: obtain written or recorded approval before additional work beyond the original quote.

Three Cs of complaint handling: listen, acknowledge the customer's concern, then explain the technical cause in plain language.

Diagnostic records should include DTCs before and after clearing, live data values, and the test that confirmed the repair.

Warranty documentation requires part numbers, invoice references, technician identification and date of fitment.

Quality check before handover: re-check fluid levels, torque critical fasteners, warning lamp status and function of the repaired system.

Customer explanation should state what failed, why it failed, what was done and what to monitor; avoid unexplained jargon.

Records retention: service and repair records are kept per statutory and warranty requirements, typically several years.