For gasoline cars, insist on a compression test: each cylinder should read within 10-15% of the others, and the actual values should match factory specs for that engine. A single low cylinder points to worn rings or valve problems that idle checks will never reveal. A leak-down test goes further. It pressurizes each cylinder and measures how much air escapes and where. Air hissing from the oil filler cap means ring wear; from the intake or exhaust means valve issues; bubbles in the radiator coolant mean a head gasket problem. This is the single most revealing test for engine condition. Also request OBD scan data with freeze-frame records, a coolant system pressure test (to catch hidden leaks), and an inspection of the timing belt or chain condition with service history. For electric vehicles, compression tests don't apply — instead ask for a battery health report from the BMS, motor insulation readings, and a road test logging any inverter or drive-unit fault codes. A written report with photos or video of each test is essential evidence for your records.

Maintain objective expectations. Vehicles over 5 years old inevitably show signs of decreased paint gloss, slight aging of rubber seals, irreversible patina on interiors, and surface rust on chassis components. As long as core structure and mechanical failures are not involved, these are normal depreciation characteristics over time.

A complete service history tells you more about a car than its mileage. In China, most maintenance is done at authorized (4S) dealers, and each visit is recorded in the brand's national database. First, ask the seller for the service booklet and maintenance invoices, then have your inspection agent call the brand's 4S network with the VIN to pull the full service record. This shows oil changes, major repairs, recall work and accident-related repairs. Second, check that the mileage recorded at each service visit increases logically. A gap between the database mileage and the dashboard reading is a red flag. Third, ask for the annual inspection (nianjian) records, which are mandatory in China and show whether the car passed emissions and safety checks on time. If the seller claims a full service history but the database shows no records, assume the car was maintained at small independent shops or neglected. Adjust your offer accordingly — plenty of used cars in China have documented histories.