What is a car diagnostic test?

A diagnostics check can help you spot potential problems with your car and fix them before they become expensive repairs.

A proactive part of car maintenance, a diagnostics check can identify issues with the engine, gearbox, brakes, exhaust and electrics. Repairing a problem that's been identified by a diagnostics check could prevent a breakdown and improve the efficiency of your vehicle. 

In this guide, we explain how a diagnostic check works, how much it will cost and what problems it may identify with your vehicle. 

What is a diagnostic test on a car?

A car diagnostic will check the health of your vehicle. The process primarily relies on the car's onboard computer system (electronic control unit, or ECU for short), which constantly monitors various functions and stores error codes when something goes wrong.

However, while the vehicle ECU will give you a warning if a serious problem has been detected - usually via the engine management light - it may not give you a comprehensive report of what has gone wrong and why. This is why a car diagnostic check is important. 

The diagnostics check will generate an error code, which makes it easy for the mechanic to drill down into the problem and the cause. In some cases, the diagnostics check will generate multiple error codes.

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What is included in a car diagnostic test?

A car diagnostic check typically includes a thorough examination of the vehicle's electronic and mechanical systems using a diagnostic tool.

The mechanic will connect a scanner to the car’s diagnostic port to retrieve error codes stored by the vehicle's computer system. Each code corresponds to a specific issue or system that needs attention.

Following this, the mechanic will then carry out the necessary fixes and maintenance and let the car owner know the relevant costs.

Once issues are identified and fixed, the mechanic may clear the error codes from the system, ensuring no further issues remain and that the vehicle's computer will continue monitoring normally.

How much is car diagnostic test in the UK?

With the RAC, drivers can expect to pay £99 when you book with one of our mobile mechanics.

Our mobile mechanics will spend up to 30 minutes checking your car for faults and talk you through any repairs you might need and give you a report for your records.

There are numerous ways we can check for faults, which could include plugging in to check your car’s onboard engine management system.

Our mechanics use parts from the manufacturer and provide a great service.

How long does a car diagnostic test take?

Car owners can expect to wait up to 90 minutes while a mechanic carries out a car diagnostic test.

However, the overall time spent at the garage may take longer if any work needs to be conducted by the mechanic.

Diagnostic error codes

These are the car diagnostic does to look out for, as they notify where the potential issue may be in the vehicle:

• B – Body. This includes issues with the air con, infotainment, airbags, and driver assistance.
• C – Chassis. This also includes the steering, the brakes, and suspension systems.
• P – Powertrain. This refers to the transmission and various engine components.
• U – wiring and sensors. This error code is linked to the wiring and sensors across the vehicle and covers other unique issues with the vehicle.

There are over 200 fault codes, so having an experience mechanic is vital, so they can make the necessary fixes or adjustments.