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Diagnose and clear fault codes with the Haynes AutoFix Electronics module

what are fault code car

Modern vehicle electrical systems are complicated things, so the first thing a workshop does when servicing a vehicle is to connect a fault code reader. If there’s a problem with the engine management or electrical systems, the vehicle may generate a fault code and store it for later retrieval by the service engineers.

You can avoid the hassle and expense of booking the car into a workshop by buying your own fault code reader, which plugs into your car's onboard diagnostic socket, also known as the OBDII. Fault code readers are readily available online and from automotive stores, and start from less than £20. They're easy to use and will reveal any stored fault codes in seconds.

However, being shown a fault code such as p0171 or p1351 is one thing, but knowing what it refers to and how expensive the fix may be is something else altogether. This is where AutoFix from Haynes comes in – it puts an electronics diagnosis module in the hands of DIY repairers for the first time. 


How to use the Electronics module in Autofix

is fault code p0171

Using AutoFix to diagnose your fault code couldn't be easier:

  1. Make a note of the fault code(s) showing on your code reader.
  2. Go to AutoFix, tap on 'Try AutoFix' and search for your car make, year, model, variant, fuel type and engine.
  3. With your purchase made, type the fault code into the 'Fault Code Search' box and press enter.
  4. The fault code translation will be shown (P0171 means 'System too lean') together with a list of affected components. Tap on each component in the list and you'll be presented with instructions on what to test for using a multimeter, including a wiring diagram (shown below) illustrating where to attach the multimeter probes. This enables you to test components in order, and establish the likely cause of a problem.
multimeter test autofix


These wiring diagrams also feature in AutoFix's Electronic's module, where you'll find illustrations for the engine management, exterior lighting, braking, central locking and many more, as well as maps of component locations and, in some cases, photos of the parts you're looking for.

Simply select the vehicle system and AutoFix will show you a list of the relevant components.

The wiring diagram is shown with colour codes, connector numbers, and even diagnosis steps where appropriate.

Our tool goes far beyond what commonly available fault code readers offer, and puts you, the car owner, back in control. Even if you don’t choose to complete the repair, you will know exactly what the problem is when you hand it over to a workshop.