What do car diagnostics tests tell you?
Tuesday 11th November 2025
A car diagnostic test gives you a clear and accurate picture of what’s happening under the bonnet. Whether a warning light has appeared or your car simply feels ‘off’, diagnostics help pinpoint the exact issue quickly. Understanding what these tests check for can give you confidence, save you money, and help you avoid major repairs later down the line.
Our guide will explore the issues that might be discovered, the importance of diagnostics, and how often your car needs to be tested.
What is a car diagnostic test?
A diagnostic test involves plugging your vehicle into specialised equipment that communicates with the car’s onboard computer system, known as the ECU (Engine Control Unit). The ECU constantly monitors sensors throughout the vehicle, searching for anything out of the ordinary. When a fault is detected, the test reveals:
- Error codes (DTCs – Diagnostic Trouble Codes)
- Affected components or systems
- The severity of the issue
- Possible causes
This means technicians can identify problems faster and more accurately than relying on just the visuals. By running regular car diagnostic tests, you can save yourself a lot of money, as they prevent small fixable issues from becoming expensive ones.
Diagnostic tools can also check your car’s computer system for manufacturer messages and stored information about the car’s history. This gives mechanics a full picture to perform the best repair possible.
What can a car diagnostic test detect?
There are 8 issues that a car diagnostic test can pick up, ranging from your car battery, exhaust and steering.
1. Engine problems
Diagnostics can pick up issues such as misfires, faulty sensors, incorrect fuel mixtures, and other underlying problems that may affect performance, efficiency and emissions. These issues often trigger the check engine light.
2. Battery and charging issues
The test can reveal whether your battery, alternator or charging system isn’t performing correctly, which is a common cause of starting problems or sudden power loss.
3. Braking system faults
Problems with ABS sensors, brake pressure levels or communication faults in the braking system can be detected before they affect your braking performance.
4. Transmission and gearbox issues
Diagnostics allow technicians to identify gearbox faults, slipping gears or issues with automatic transmission sensors that would otherwise go unnoticed until the fault becomes severe.
5. Exhaust and emissions problems
Sensors within the exhaust system monitor emissions constantly. Sensors such as Lambda sensors (which monitor oxygen levels), NOx (Nitrogen Oxide) sensors and Exhaust Gas Temperature (EGT) sensors will relay information back to the ECU. These tests pinpoint any issues affecting emission levels and fuel economy.
6. Airbag and safety system errors
If there’s a fault with airbags, crash sensors or seatbelt tensioners, diagnostics can detect it and flag it long before you notice anything is wrong.
7. Steering and suspension faults
While physical inspections are still essential, diagnostics can detect electronic issues within power steering systems or suspension components that use digital sensors.
8. Electrical system malfunctions
From failing modules to faulty wiring, diagnostics can trace electrical problems that cause dashboard warning lights, miscommunication between systems or intermittent behaviour.
Why are diagnostic tests important?
Diagnostic tests help you identify issues early, often before a major breakdown occurs. Small problems, like a misfiring spark plug or a failing sensor, can lead to bigger and more expensive repairs if ignored. Essentially, diagnostics provide peace of mind and give you a clear understanding of your car’s health.
Do you need a diagnostic test even without a warning light?
Yes, you don’t need a warning light to have a diagnostic test. Not all faults trigger dashboard warnings. If your car feels sluggish, makes unusual noises, or is using more fuel than normal, a diagnostic check can reveal underlying issues that the ECU hasn’t flagged yet.
How often should I get a car diagnostic test?
Ideally, you should get a new diagnostic test every year to be on the safe side. However, if you feel there is something wrong with your car, do not hesitate to get a diagnostic test done right away. You can also utilise car diagnostic tests as a useful tool for checking if your recently purchased car is in good condition.
Book a Diagnostic Test at Formula One Autocentres
If something doesn’t feel right with your car, or if a warning light has appeared, servicing can help. At Formula One Autocentres, our expert technicians use advanced diagnostic equipment to identify faults and recommend the right repair. If you’re in need of a?car diagnostic test, we offer diagnostic checks as part of a service, find your local Formula One Autocentre?for a car check-up.
Need more information??Contact our helpful team?today.
FAQs
How long does a car diagnostic test take?
Most diagnostic tests take around 30 minutes, depending on how many error codes are detected and whether a deeper investigation is needed.
Will a diagnostic test fix the problem?
No. Diagnostics identify the problem. Repairs will then be recommended based on the results.
Can I drive with a warning light on?
Some warning lights indicate minor issues, but others, especially engine, brake or airbag lights, require immediate attention. A diagnostic test will confirm how serious the problem is.
