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?
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.
Several types of diagnostic tests exist to help you pinpoint issues with your vehicle. The classic test involves plugging an ECU reader into your car’s OBD port to show fault codes, test components, and view live data. Other test types include smoke tests for air leaks and visual checks performed when the car is on a ramp. A proper diagnostic test can help minimise vehicle off-road (VOR) time. If used correctly, it will save you money by ensuring only necessary repairs are carried out.
Matt Wrankmore
Head of FixMyCar's Garage Network
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).
iStock.com/Drazen Zigic 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.
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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.
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
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