OBD-II became a legal requirement for all cars sold in the United States from 1996. Any car from that year onward has an OBD-II port, and any device that connects to it can read the fault codes and live sensor data stored in the engine management system.
A basic OBD-II reader costs $30. A quality scan tool with live data display costs $200 to $400. For a pre-purchase inspection of any modern classic, connecting a scan tool should be standard.
Active faults versus stored faults
OBD-II stores two categories of fault codes. Active faults are currently triggering a warning light or condition. Stored faults are issues that occurred at some point in the car's history and were either resolved or cleared.
Any car you are seriously considering should have no active fault codes. Active faults with no explanation are a reason to either delay the purchase until they are resolved or decline entirely.
Stored faults require context. A stored fault for an oxygen sensor that was replaced six months ago is expected and benign. A stored fault for the same issue that has never been addressed is different. Ask the seller about any stored faults you find.
Codes that matter
P0xxx codes relate to the powertrain. P0300 to P0312 are misfire codes. A misfire on a specific cylinder can have many causes: a faulty ignition coil, a bad spark plug, a fuel injector issue, or in more serious cases a mechanical problem like low compression.
P0420 and P0430 relate to catalytic converter efficiency. On a high-mileage car, these codes can indicate a catalyst approaching end of life. P0xxx transmission codes are particularly significant and can indicate anything from a minor sensor issue to serious internal wear.
Manufacturer-specific codes
The basic OBD-II code set is standardised. Manufacturer-specific codes, which appear with prefixes like P1xxx or U1xxx, require a manufacturer-specific scan tool to read and interpret correctly. A basic $30 OBD reader will miss these entirely. For a Porsche, Ferrari, BMW, or any other specialist car, you need either the manufacturer's own diagnostic equipment or a third-party equivalent.
Cleared codes
Some sellers clear fault codes before a sale. There is one way to detect this: ready monitors. When codes are cleared, all monitors reset to "not ready". A car with all monitors showing "not ready" has had its codes recently cleared. This is a flag that warrants a direct conversation with the seller and a drive cycle to allow monitors to reset and see whether any codes return.
Live data
Beyond fault codes, a quality scan tool provides live sensor data. Short-term and long-term fuel trim values that are significantly positive or negative indicate the engine is compensating for a rich or lean condition. A car running at normal fuel trim is a much better signal than one that is compensating heavily.
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