Experience has shown me it’s best to look for clues and ask some questions if needed before testing components. I’ll look for signs of recent accident damage, water leaks, heavy corrosion, or recent works carried out on the vehicle. A scan tool commonly won’t communicate because one faulty module pulls the whole system down. A damaged circuit is common too, but not as common as a failed module.

An OBD reader has traditionally taken the form of a handheld diagnostic computer that communicates with your car’s brain to diagnose fault codes, among other things. You can pay as little as a tenner for an OBD scanner and as much as hundreds of pounds – with the latter offering a multitude of features – but whichever you choose, they should all have the same minimum requirements, and be able to:A technician may describe soft codes as historical anomalies or intermittent codes that don’t affect vehicle function. Draper has a good reputation for making products that stand the test of time, and we have no reason to doubt this code reader's staying power; at the time of testing it was one of the pricier offerings here, but deserves its Haynes Recommended award. Made for all kinds of users, from professionals to home mechanics, this industry standard OBD II reader pinpoints why the vehicle’s engine light is showing. The tool also runs a number of diagnostic tests such as the O2 sensor test, I/M readiness test and EVAP test. It also has permanent code retrieval, on-board monitor test (Mode 6, for additional component/system monitoring), reads fault code pending data, DTC code lookup function and lets you reset the oil change indicator.

While total communication loss is rare, it’s more usual for one module to go down, and the other working modules report its loss by setting a fault code. CAN employ a twisted pair of wires (one wire CAN high typically 3.5-2.5 volts oscillating and the other CAN low typically 2.5 – 1.5 volts oscillating) that loop the vehicle, connecting all the modules on the network.

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Please note: The reader will not be able to read codes for airbags and braking systems. OBD II readers mainly read OBD II codes (emissions systems) in 99.9% of vehicles. So if the cheapest wired scanner – the Motorpower – is capable of scooping top honours, can the lowest-priced device in this test also achieve great things? The reader can also determine the malfunction indicator lamp status, review emission readiness status, retrieve VIN (Vehicle Identification Number) on 2002 and newer vehicles that support Mode 9 and more. Wireless readers are smaller and generally cheaper, but need a connection to a smartphone or tablet. Wired readers are more convenient, easier to use with gloves and grimy hands, but usually provide less information. We tested both.

The reader requires no battery, it plugs straight into the vehicle’s connection point for it to power up. It is compatible with all petrol vehicles from 1996 and diesel vehicles from 2004 that are OBD II compliant (including CAN). All these modules (aka nodes) communicate with each other using a system known as CAN (Controller Area Network).Scan tool faults aren’t that common, I’ve never broken a tool yet, but I have worn out a few cable connectors. Here’s a list of scan tool problems and what you can do to fix them: This reader’s large colour display looks like it should be a touchscreen, but it’s not. However, the physical buttons make navigation a cinch.