What Is a Car Diagnostic Test & Why Should You Book One?

Ellie Dyer-Brown, 17 days ago

5 min read

  • Diagnostic
  • Car ownership
Car diagnostic test

FixMyCar explains what a diagnostic test is and what it can tell you about your car.

Modern vehicles are highly computerised, which has led to advances in safety and performance. Another significant benefit is that special software can be used to identify existing and developing problems - that’s how a car diagnostic test works.

Unfortunately, many drivers are unsure what a diagnostic test can tell them, what it covers and what benefits it can bring. That’s where this guide comes in. Read on to find out everything you need to know about diagnostic tests for cars.


What is a car diagnostic test?

How do car diagnostic tests work?

What parts of a car are tested?

How often are car diagnostic tests needed?

How long does it take to do a car diagnostic test?

What are the benefits of car diagnostics?

Book a diagnostic test

What is a car diagnostic test?

A diagnostic test uses specialist software to check whether your car and its components are functioning correctly. Getting your vehicle tested allows problems to be caught early before they become more severe and more expensive to repair.

How do car diagnostic tests work?

A diagnostic test digitally analyses your car's computer systems and components. Issues are detected using data from sensors, built-in processors and microchips, all of which are monitored by the engine control unit (ECU).

Car diagnostic test

When the ECU detects a problem, it typically triggers a dashboard warning light and an error code. At this point, a diagnostic test is used to read the code and determine what has gone wrong. But you don’t need an illuminated warning light to book a diagnostic test; regular testing can identify issues before they become severe enough to trigger an ECU error code.

What parts of a car are tested?

A diagnostic test will report on various components and systems, including:

  • Engine

  • Transmission

  • Brake responsiveness

  • Exhaust faults or contamination

  • Performance issues

  • Damaged or faulty components, such as the throttle, ignition coils and fuel injectors

How often are car diagnostic tests needed?

Most drivers wait for a dashboard warning light to book a diagnostic test. However, diagnostics can also be used as a preventative measure. 

Diagnostic tests aren’t like MOTs - you don’t need to book them according to a schedule, and they’re not technically ‘needed’ in the traditional sense. But they are valuable tools for finding issues quickly and efficiently.

If you think there might be something wrong with your car but you’re not sure what it is, we’d recommend getting a diagnostic test rather than guessing what the problem is and booking a repair that might turn out to be wrong.

How long does it take to do a car diagnostic test?

A comprehensive diagnostic test will take about an hour. In some cases, problems might be discovered that make the process more complex, or components may have to be removed before the test can be carried out, which results in it taking longer.

Technician doing a car diagnostic

How much does it cost for a car diagnostic test?

The average price of a diagnostic test is £51.66. The table below shows how prices vary between different manufacturers.

MakeAverage price

Peugeot£51.73 Renault|£51.03 SEAT|£49.87 Skoda|£51.36 Suzuki|£51.72 Toyota|£50.11 Vauxhall|£52.44 Volkswagen|£53.42

What are the benefits of car diagnostics?

Diagnostic tests are incredibly valuable for the following reasons:

  • Detecting problems before they become severe will save you money in the long run.

  • Catching issues early could save your life and the lives of other road users - for instance, by picking up on brake problems before your brakes fail on the motorway.

  • Diagnostic tests can also check your ECU for historical data and manufacturer notifications, which is particularly helpful if you’re buying a used car.

  • Timely repairs and regular maintenance will help maintain your car’s performance.

Book a car diagnostic test

FixMyCar can help you find a reliable local mechanic.

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Written by Ellie

Ellie Author Pic

Ellie is FixMyCar’s Content Writer. She has a BA in English literature from Durham University, a master’s degree in creative writing, and three years of experience writing in the automotive industry. She currently drives a Suzuki Swift.

Find Ellie on LinkedIn.