Professional diagnostics of fault codes start at €29, but with an OBD-II (On-Board Diagnostics) scanner you can fix many issues yourself. You can clear temporary faults with just a few clicks, and identify structural problems before they lead to expensive repairs. Curious how to get started reading fault codes yourself?
What is OBD diagnostics and how does it work?
OBD stands for On-Board Diagnostics and is a standardised system in modern cars. It monitors electronic systems such as engine management, emissions and transmission. Since 2001 all petrol cars in Europe have been required to have OBD-II, diesels since 2004. This system stores faults as fault codes that you can read via the OBD-II port.
You will usually find the OBD-II port under the dashboard on the driver’s side or in the centre console. In models such as the Ford B-Max the port is often located to the left under the steering wheel. With a scanner you can see all active and stored fault codes within seconds.
The system uses DTCs (Diagnostic Trouble Codes), fault codes that describe malfunctions in a globally agreed code system. Each code consists of a letter followed by four digits that describe the fault. This makes diagnosis faster and more accessible for both professionals and private owners.
How do you read fault codes yourself?
Reading fault codes is done in a few simple steps. You will need an OBD-II scanner, available from €20. Follow this approach:
- Switch the ignition on without starting the engine
- Connect the OBD-II scanner to the diagnostic port
- Start the scan via the device or the accompanying app
- View the fault codes that have been read and write them down
- Look up the meaning of each code for further analysis
Modern scanners often work via Bluetooth and pair with a smartphone app. This offers advantages such as live data display and automatic code interpretation. Standalone scanners work without a phone and are handy for people who do not want to use an app.
Note that basic scanners mainly read engine faults. For ABS problems, airbag faults or transmission errors you often need more advanced equipment. Make-specific tools such as FORScan for Ford models offer more detailed fault reading than universal scanners.
Different types of fault codes and what they mean
Fault codes start with a letter that indicates the system. P stands for powertrain, B for body, C for chassis and U for network (communication between modules). The four digits that follow specify the fault.
There are three categories of fault codes. Active fault codes point to a malfunction that is currently present, usually accompanied by a warning light. History faults have occurred in the past but are no longer active. Intermittent faults occur sporadically and are therefore difficult to diagnose.
Common codes you may encounter:
- P0300: random misfire, often caused by worn spark plugs or ignition coils
- P0420: catalytic converter problem, usually after 100,000 kilometres
- P0171: mixture too lean, often due to a dirty mass air flow sensor
- P0301–P0304: misfire in a specific cylinder, indicates a coil or spark plug fault
The first digit after the letter indicates whether the code is generic (0) or manufacturer specific (1). This explains why some codes only occur with certain manufacturers.
Which scanner suits your situation?
The choice of scanner depends on your technical knowledge and budget. Bluetooth scanners between €20 and €50 are ideal for basic diagnostics. They pair with free or inexpensive apps such as Torque or Carly and show, in addition to fault codes, live data such as engine speed and coolant temperature.
Standalone scanners cost between €50 and €150 and work without a smartphone. They are more robust and more convenient for use in the workshop. Many models have a display that shows code descriptions directly and offers the option to clear fault codes.
Professional make-specific tools such as FORScan start from €100 but offer extensive capabilities. They not only read engine fault codes but also ABS, airbag, climate control and other modules. For anyone who regularly works on cars or owns several vehicles of the same make, this is a valuable investment.
Can I clear faults myself and when is it safe?
You can clear fault codes on most scanners at the press of a button. This resets the engine management system and switches off warning lights. Only clear codes when you are sure the underlying cause has been resolved, otherwise they will return within a few kilometres.
Temporary faults sometimes disappear by themselves after disconnecting the battery for a few minutes. This can work with one-off sensor glitches or software hiccups. Structural problems such as a defective lambda sensor or catalytic converter will not be solved by clearing codes.
Be careful when clearing certain fault codes:
- Airbag faults often require professional recoding
- ABS faults can affect safety
- Transmission faults in automatics often require additional diagnostics
- Emissions problems can lead to MOT failure
After clearing codes it is advisable to take a test drive. This lets you check whether the fault returns and whether all systems are functioning correctly. Some cars need a so-called readiness period before all sensors are recalibrated.
Where can you get low-cost professional diagnostics?
Garages and car dealers charge between €29 and €100 for reading fault codes. This fee usually only covers the diagnosis; repair costs are extra. Chains such as Kwik-Fit and ATU sometimes offer cheaper rates or free quick scans during promotions.
Doing diagnostics yourself saves these costs entirely. A good scanner pays for itself within a few reading sessions. In addition, you have the freedom to check faults immediately when a warning light appears, without having to make an appointment.
Consider professional help for complex faults that require deeper knowledge. Problems with the CAN bus system (the communication network between electronic components in the car), interference between modules or intermittent electrical faults are difficult to tackle without experience and advanced equipment. A good middle ground is to read codes yourself and have a specialist verify them.
Practical tips for effective fault-code diagnostics
Always start by recording all fault codes before you clear anything. Also note under which conditions the problem occurred: on a cold start, while driving or after prolonged use. This context helps you uncover the real cause.
Use online forums and databases to read about other owners’ experiences. Many makes have typical weak points that repeatedly generate the same fault codes. For models with sliding doors, for example, problems with door modules are well known, which standard scanners do not always detect.
Invest time in understanding live data. A scanner that shows parameters such as fuel pressure, timing and lambda values gives more insight than fault codes alone. This helps you distinguish between symptoms and causes. A P0171 code may point to a leaking intake hose, but live data shows whether the mass air flow sensor is also giving abnormal values.
Keep a logbook of all fault codes and repairs carried out. This builds up a history that helps with future problems and increases the resale value. Potential buyers appreciate transparency about maintenance and repairs.
‘ Discover how to carry out simple repairs yourself and when professional help is needed. ’
Discover more about car maintenance and trouble-free driving
Fault-code diagnostics is one of many ways to keep your car reliable and avoid unexpected costs. On the Carnews website you will find extensive information on common car problems, maintenance schedules and practical tips for different makes and models. Discover how to carry out simple repairs yourself and when professional help is needed.
Frequently asked questions
For a car enthusiast, a Bluetooth OBD-II dongle with smartphone app is usually the best choice, because it lets you see live data, fault codes and extra sensor information for a relatively low price. Choose a reliable ELM327-compatible adapter (not the very cheapest knock-off) in combination with a good app such as Torque Pro or, if you have a Ford, FORScan. If you prefer not to use a phone, a standalone hand-held scanner of around €50–€150 is suitable, with a clear screen and support for live data. If you mainly drive one make (for example Ford), make-specific software or a make-specific scanner is the best option for in-depth diagnosis of ABS, airbag and transmission.
You read fault codes via the OBD2 port (usually under the dashboard) with an OBD2 scanner or Bluetooth dongle with app; switch the ignition on, connect the scanner and let the system read out the DTC codes (for example P0301, P0420). The letter indicates the system (P = engine/gearbox, B = body, C = chassis, U = network) and the digits specify the fault, which you then look up in the app or online. Active fault codes point to a current malfunction, stored or intermittent codes to earlier or sporadic problems. Only clear faults once the cause has been eliminated, otherwise the warnings will return.
You can usually diagnose fault codes yourself if they relate to engine management or simple sensors, the car still drives normally and no safety-related lights (such as airbag or ABS) are lit. Use an OBD2 scanner and check whether the complaint is mild (for example rough running, increased fuel consumption). Go straight to a specialist if airbag, ABS, brake or transmission lights are on, if the car goes into limp-home mode or feels unsafe, or if the same fault code returns immediately after clearing. If you are in any doubt, or there is smoke, a strong smell or severe loss of power, always involve a professional immediately.
Common, simple fault codes around engine management (such as slight mixture deviations or occasional misfire codes) can often be tackled yourself by, for example, checking spark plugs or sensors and then clearing the code. Recurring engine faults, catalytic converter codes and everything related to ABS, airbags, transmission or network (P0420, B and C codes, U codes) usually require professional diagnosis. For safety reasons you should never clear or ignore airbag and brake-related fault codes yourself. If a fault code keeps coming back after your intervention, professional help is always necessary.
Use an OBD2 scanner to read fault codes as soon as the ‘check engine’ light comes on and note both the code and the circumstances (mileage, driving conditions). Look up the meaning of the code and tackle the underlying cause straight away (for example cleaning sensors, replacing worn parts) instead of just clearing the code. Read the car occasionally as a preventive measure, even without a warning light, so that you can detect early or intermittent faults in good time. Keep a logbook of fault codes and repairs carried out to recognise patterns and prevent future problems.