Although a vehicle examiner considered your vehicle safe to drive and issued you with a roadworthy or safety certificate, your vehicle may still have defects and shortcomings in systems and components that generally do not affect your safety or the safety of other road users.
However, while these types of shortcomings or defects will generally, but not always, make your vehicle unsafe to drive, your vehicle may, on the other hand, have defects and shortcomings you may not be aware of, and which could cause it to fail an inspection on multiple counts. With that in mind, consider the list below of issues that might cause your vehicle to fail a roadworthy or safety inspection, but note that this list only contains the most common reasons, as opposed to all possible reasons a vehicle might fail an inspection--
Fluid leaks
The most common leaks are oil leaks from valve/tappet covers, main bearing seals, crankshaft/camshaft seals, timing covers, and oil sumps. Oil leaks are closely followed by engine coolant leaks from loose hose clamps radiator tanks, with brake fluid leaks from master cylinders and wheel slave cylinders.
Next, come transmission fluid leaks from various sites on automatic transmissions and hydraulic power steering fluid leaks from worn or damaged hoses or power steering rack seals.
Brakes
Many vehicles fail inspections because the brake pads are excessively worn, or the brake rotors are deeply scored and/or discoloured, which is evidence that the brakes had overheated at some point. Other brake system issues include uneven brake performance on the same axle, or the hand/parking/emergency brakes not holding the vehicle stationary on a prescribed incline.
Suspension system and steering defects
These include worn or damaged shock absorbers, worn, broken, or damaged suspension bushings, broken springs and/or shackles, as well as worn ball joints, tie-rod ends, and drag links.
Tyre and wheel issues
These typically include worn, or unevenly worn tyres, or tyres that are visibly damaged in any way. Note also that although a vehicle will usually not fail an inspection if the tyres are of different brands, the vehicle will almost certainly fail the inspection if the tyres (including the spare tyre) do not have the same (or correct) weight, pressure, traction, and speed ratings.
Exhaust system issues
If there are exhaust leaks present, the vehicle will almost automatically fail the inspection. In addition, a vehicle may also fail an inspection of the bracketry and rubber hangers that secure the exhaust system to the vehicle are in a less than perfect condition.
Some types of exhaust modifications, such as the removal of catalytic converters and silencers will also cause a vehicle to fail an inspection.
Windscreen and glass tinting
Windscreens (and any other glass on the vehicle) that are cracked, chipped, or otherwise damaged will almost certainly cause the vehicle to fail the inspection.
However, as far as window tinting goes, excessive tints cause as many, if not more vehicles to fail vehicle inspections across Australia than cracked or chipped windscreens. In practice,each state or territory has its own maximum allowable window tint, or VLT (Vehicle Light Transmission) laws/regulations, so be sure to research the window tinting regulations in your state or territory before you commit to a roadworthy or safety inspection that your vehicle may very well fail because its windows might be too heavily tinted.
Lighting
Typical lighting issues include headlight, tail/brake light, indicator lights not working, or head/taillight lenses that are cracked, broken, faded, or clouded over. In addition, headlights that do not switch from high to low beam, or vice versa for any reason is a common reason why many light vehicles fail roadworthy or safety inspections.
Note that even if your headlights work perfectly, but are not aligned properly, your vehicle will also fail an inspection.