A pre-purchase inspection report quantifies the condition of a vehicle instead of giving an opinion of whether it’s worth buying.
Each measurement has a manufacturer specification, an acceptable limit, and an estimated repair cost if left neglected. Armed with this information, it’s easier to tell if a vehicle needs to have wear items replaced or if it’s in need of repair for other faults.
Brake Components: Pad Thickness and Rotor Minimum
Brake pad thickness is given in millimeters. While the measurement will differ from manufacturer to manufacturer, most will recommend replacement when the pad thickness is around 3 mm.
New pads are typically 10-12 mm. Letting the pads wear down to the limit can cause the pad backing to damage the rotors, resulting in a greatly increased repair cost instead of a simple pad replacement.
Rotor minimum thickness is set by the manufacturer and will differ for each specific set of rotors. The Federal Motor Vehicle Safety Standards require brake components to function within the manufacturer’s specifications.
A rotor worn to or below the minimum thickness can’t reject as much heat as a fresh rotor. Driving on rotors this thin can also reduce braking performance and reduce the lifespan of a set of pads.
Tires: Tread Depth and DOT Date Code
Tire tread depth has a large effect on wet traction and stopping ability. The U.S. Department of Transportation (DOT) sets the legal minimum at 2/32 inch or 1.6 mm.
While this is fine for dry traction and is the measurement at which a tire is considered “bald” by tire retailers and most state inspections, most manufacturers recommend replacement at 4/32 inch for adequate wet traction and 5/32 inch for winter weather traction.
While many vehicles will pass an inspection with worn tires, driving with tires this low in tread will greatly increase the risk of hydroplaning and could mandate the replacement of the tires within a matter of months.
Increasing year-round performance limits could potentially save a driver’s life in an emergency situation.
Engine Health: Compression and Leak-Down
Compression testing measures the pressure left in a cylinder that can be created by the compression of the air-fuel mixture in the cylinder and the rocking force of the pistons.
While acceptable ranges can vary greatly between manufacturers, most specifications require that all cylinders be within about 10% of each other, PSI-wise. A lower reading indicates that the cylinder may have worn piston rings or have a leaking valve or head gasket damage.
Leak-down over 20% is bad and can come from a number of areas, but is usually related to internal wear. Repairs for cylinders with over 20% leak-down can include anything from valve refacing work to a non-factory repair, rebuilding of the engine’s short block in the next year.
This can easily increase the vehicle’s cost and negate any benefits of a bargain purchase in a matter of months.
Brake Fluid, Battery Performance, Readiness Monitors
Brake fluid absorbs water. The Society of Automotive Engineers and brake fluid manufacturers indicate that fluid with a water content of 3 per cent provides poor resistance to boil-over and increases the opportunity for corrosion.
With fluid reasonably inexpensive to change, it would be a shame to destroy the hydraulic components that the fluid bathes.
Though many think of battery condition in terms of voltage, it is most commonly assessed through Cold Cranking Amps (CCA). The testing device will measure output, with the vehicle’s rating offered by the manufacturer.
Batteries that fall well below this rating tend to struggle in the winter. New battery sales usually get a bump in the fall, before a long winter. OBD-II readiness monitors are a great way to determine that emissions functions have passed their self-checks after a repair or battery replacement; however, most forget that incomplete monitors can prevent a car from simply passing an emissions inspection.
In other words, you might find that there is nothing wrong with the vehicle, but it still failed emissions due to an OBD fault. Well, the fault here is not in the vehicle; it is in the customer.
In this case, whoever is responsible for the sale should have completed a simple 10-minute operation to be ready for a car inspection in Philadelphia.
Wear Limits at a Glance
| Measurement | Service Limit | One-Year Cost Only if Ignored |
| Brake pads | Approx. 3 mm | Pads and rotors |
| Rotor thickness | Minimum stamped on rotor body | Rotors |
| Tyre tread | 2/32 inch legal minimum | Tyres |
| Compression variance | Above 10% between cylinders | Diagnosis |
| Leak-down | Above 20% | None |
| Brake fluid moisture | Approx. 3% | None |
| Battery CCA | Well below rated value if new | New battery |
| OBD-II readiness | Incomplete monitors | None |
Pennsylvania requires annual safety inspections covering braking, tyres, steering and suspension, lighting, and other systems. Vehicles in the Philadelphia area must also pass emissions testing.
OBD-II testing is required for most gasoline vehicles manufactured in 1996 or newer.