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.

VIN consistency is checked at customs in both China and your country, and a mismatch can block clearance. Do the following: 1. Verify early: before payment, ask for clear photos of the VIN plate (dashboard, door frame, engine bay on some models) and compare them with the VIN on the purchase contract, inspection report and any registration documents. 2. Check the characters: VINs never contain the letters I, O or Q. If you see them, the document is either fake or mistyped. 3. Decode it: the first 3 characters are the world manufacturer code, characters 4-8 describe the model and configuration. Decode it online to confirm the documents match the actual trim level you are buying. 4. At destination: when the car arrives, re-check the VIN against the bill of lading and your import declaration before accepting delivery. Any discrepancy found before shipping can be corrected cheaply; after arrival it becomes a legal problem. Make VIN verification part of your pre-shipment inspection checklist.

It's extremely difficult. Unauthorized modifications can lead to module current overload protection, resulting in a lock. In China, you need to use the factory online engineer account to reprogram it. It's basically impossible to perform online decoding overseas. We strongly warn against blindly modifying the electrical systems of modern integrated circuit vehicles.