Workshop Basics

How to talk to your modern classic’s On Board Diagnostics system

Become OBD literate.

On Board Diagnostics – or OBD – can strike fear into the heart of a classic driver brought up on carburettors and contact breakers. Panic sets in when the engine management light flashes on. In reality, though, OBD is our friend. If you can read your car’s OBD system, fault-finding becomes much easier. Buy a basic fault-code reader and OBD turns into a real blessing.

OBD has been around since the Eighties, when manufacturers embraced advances in electronic systems. Performance cars adopted digital engine management and electronic fuel injectors, which controlled fuelling and ignition far more accurately than previously possible. Spark timing and fuelling controlled dynamically by an Electronic Control Unit – or ECU – improved both performance and fuel economy. Critically, an ECU and exhaust catalyst could also reduce emissions to meet demanding standards imposed across the industry. Soon even the most commonplace cars were using the new technology.

The ECU relies on inputs from sensors dotted around the engine. These include air temperature, coolant temperature, manifold air pressure, induction airflow, throttle position, knock detection and exhaust oxygen content. The ECU makes decisions based on the information from these sensors using pre-programmed sets of instructions known as a ‘maps’. This is wonderful as long as all the aforementioned components work correctly. If a sensor develops a fault, there’s potential for the engine to run badly or stop altogether.

If the ECU detects a sensor input outside its expected range, it’ll store a faultcode and possibly illuminate a warning light on the dashboard. It may choose to ignore the sensor and switch to ‘limp home’ mode.

A cheap faultcode reader – or ‘scanner’ – is all you need to find out what’s wrong. This guide covers the basic principles. We’re starting with OBD1, which was specific to each manufacturer and required unique scanners and protocols. We’ll then look at OBD2, a sophisticated universal system that took over in the mid-nineties.

You will need Scanner capability Short cuts? Connections OBD scanner and a laptop, mobile phone or tablet with suitable software; multimeter; electrical contact spray. Many OBD1 cars will need an adaptor to connect to the scanner. Cheap scanners should diagnose and clear engine faults, but are unlikely to deal with security system, airbag or ABS problems. Advice on the internet may suggest shorting out pins in the OBD socket to reset the service indicator. This is best avoided, as mistakes can damage the ECU.