The modern car is more than just oil and steel. Electronics help manage engine performance, regulate braking and steering and control safety features like seatbelts. If something goes wrong with the electrical system or sensors that monitor fuel economy, a computer will alert you by turning on the check engine light (and if it’s serious, may set off other alarms). A obd car scanner can read that warning and tell you what’s the problem. The most expensive of our picks can even offer a description of the issue, but less costly models will simply give you a numeric code that you have to look up online or in your owner’s manual.

These gadgets plug into a 16-pin diagnostic socket on the vehicle’s underhood (called an OBD port in America and EOBD in Europe). They are designed to work with all passenger cars built since 1996, so most models should be compatible. Some are stand-alone units, but others are a USB or Bluetooth dongle that attaches to your laptop or smartphone.

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Our top pick is a dedicated device that offers a clear, easy-to-use display and hot keys for the most frequently used functions. It’s also packed in a rugged case and comes with a thorough manual, a one-year warranty and US-based customer support. We also recommend Draper’s cheaper model, which has a small but decent-sized screen and intuitive menus that let you see live data and access pending and current codes, mode 06 test results, and freeze frame data. It also has a feature that lets you update its software and a USB lead for transferring data to your PC.