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How to check and diagnose a car’s common fault codes

About the author

Rob Keenan is the interim digital editor of Haynes.com
He runs a Mk2 Ford Focus ST and an ageing Mercedes SLK55
Find him on Twitter @zorba_t_greek

where is OBD port

Had a check engine light appear on the dashboard? It's easy to fear the worst when a 'major' warning light appears, but it's not always necessarily going to cost big bucks to sort.

Traditionally, you'd have to take your car to a main dealer for them to plug their expensive diagnostic equipment into your car's brain to figure out what's going on. But nowadays you can pick up an OBD code reader for as little as £20, and often less.

What's an OBD port? OBD stands for on-board diagnostics and the port looks a little like the SCART socket you used to get on older TVs and video equipment. The port is usually located beneath your car's steering wheel, but it could be behind a trim panel or elsewhere - your Haynes manual will show you where yours is.

Read more with The Haynes Manual on Fault Codes.

The video below shows how to use an OBD code reader on a typical car and discusses three fault codes:

Fault code P0001

Indicates a problem with the circuit running from the engine control module (ECM) to the fuel pressure regulator on your fuel injection rail.

Fault code P0002 and P0003

Similar to the above, these issues concern the ECM itself and a problem with its ability to read signals from the fuel pressure regulator.

Watch this to see how to use an OBD code reader

Every Haynes manual shows you how to locate and use your OBD port in easy-to-follow steps