Reading fault codes is the basis of modern vehicle diagnostics and helps you quickly identify underlying problems. In commercial vehicles such as the Citroen Jumper there are often more than twenty different stored fault codes, each pointing to specific irregularities in the system. With the right diagnostic equipment you can bring these codes into view. That gives you a clear starting point for further analysis. This way you avoid working blind when dealing with faults and you can work in a targeted way.
A fault code is not a final diagnosis. It is a signal: a particular system or sensor has registered an irregularity. It indicates where the vehicle detects a problem, but not always which component is actually defective. Therefore interpret the code in combination with the complaint, the driving behaviour and other symptoms. A systematic approach prevents unnecessary replacement of parts and saves time and money.
Diagnosis is all about organising complaints. You check patterns. You rule out alternative causes. You achieve the best results by combining technical checks with the driver’s observations. In this article you will read how to read fault codes yourself and which equipment you need for this. You will also read why resetting is important. Finally, we discuss codes that require immediate attention.
What is an OBD scanner and how does it work
An OBD scanner is a device. It enables communication between your car and a diagnostic reader. On-board diagnostics – the system in modern vehicles that records and stores faults – is abbreviated to OBD. You will usually find the OBD port under the dashboard on the driver’s side. You connect to this port with a scanner or laptop. You can then retrieve data.
Different types of scanners are available:
- Simple code readers that only display fault codes
- More advanced scanners with live data and parameter display
- Professional diagnostic tools that provide access to all control units and actuators
- Bluetooth adapters that work with an app on your smartphone
Most modern vehicles have a standardised connection, which has been mandatory in Europe since 2001 for petrol cars and since 2004 for diesels. This is the so-called OBD-II port. This standard makes reading fault codes accessible for private users. You no longer have to go straight to the garage for an initial diagnosis.
How to read fault codes yourself
You read fault codes in a number of steps. Start by locating the OBD port in your car. It is usually located under the steering wheel, near the fuse box. Sometimes it is hidden behind a little cover on the centre console. Consult the manual if you cannot immediately find the port.
Then follow this procedure:
- Switch on the ignition without starting the engine
- Connect the OBD scanner to the port
- Wait until the scanner has established a connection
- Select the option to read fault codes
- Write down the displayed codes and any descriptions
- Optionally, view live data such as engine temperature, engine speed or sensor values
Fault codes usually consist of a letter followed by four digits. The letter indicates the system. The first digit shows whether it is a generic or manufacturer-specific code. The remaining digits indicate the specific component or system. P stands for powertrain (engine and transmission), C for chassis, B for body and U for network. A code such as P0171, for example, points to a mixture that is too lean in the engine.
Why you should reset fault codes after a repair
Clear fault codes to check whether a repair has been successful. After you have replaced a component or resolved a fault, the old code remains stored in the memory. The warning light on the dashboard may stay on if you do not clear the code. You then cannot be sure whether the problem has actually been solved.
After resetting, the system starts monitoring again. If the fault is still present, the code will reappear after a number of driving cycles. This confirms that the root cause has not been completely resolved. It also prevents you from confusing old codes with new problems, which would cloud the diagnosis.
Some systems go through a learning phase after clearing codes. Only then do all functions work optimally again. This applies, among other things, to the engine management system. This system adapts to driving behaviour and conditions. After resetting, drive calmly for a number of kilometres to allow the systems to recalibrate.
Which fault codes are critical and require immediate attention
Not all fault codes are equally important. Some indicate a minor deviation that you can have checked later, while others require immediate action. Codes related to safety, exhaust emissions or engine damage should be prioritised.
The following categories require quick action:
- Codes relating to the braking system or ABS
- Problems with the airbag or seatbelt pretensioners
- Engine faults that can lead to overheating
- Serious emission deviations that cause damage to the catalytic converter or diesel particulate filter
- Problems with fuel pressure or injection
- Defects in the cooling system
Be alert if there is greatly altered driving behaviour, strange noises, smoke or smells. In that case, stop driving. Have the vehicle checked. In such situations, continuing to drive can lead to additional damage or an unsafe situation. Codes that only relate to sensor values or minor deviations in the emission system are often less urgent, but should still be resolved within a reasonable time.
‘ The real cause is often somewhere else. ’
How to interpret fault codes in combination with symptoms
A fault code gives direction. It does not tell the whole story. Link the code to the complaint and the driving behaviour. Suppose a code points to a lambda sensor. The cause may be a defective sensor. But also consider an exhaust leak or a problem with the fuel supply.
Describe the complaint as precisely as possible. Does the problem occur with a cold engine? Or only after driving for a long time? Does it occur at low speed or under load? Also pay attention to certain weather conditions or situations after refuelling. This information helps you or the mechanic to rule out alternative causes. Many faults first produce vague symptoms such as loss of power, juddering during acceleration or an irregular idle speed.
First check simple causes. Think of loose cables, dirty contacts, a flat battery or clogged filters. Do this before you replace expensive parts. Do not immediately replace a sensor or actuator based on a single fault code. The real cause is often somewhere else. Keep a logbook of complaints, conditions and codes. This will help you recognise patterns in recurring problems.
What to do if fault codes keep coming back
Recurring codes indicate that the underlying cause has not been resolved. This occurs regularly. Often only the symptom has been addressed instead of the real culprit. A replaced component may be fine in itself. However, the problem may lie in the wiring, another sensor or a mechanical defect.
In that case, take the following steps:
- Check the wiring and connectors of the relevant system for damage or corrosion
- See whether several stored codes are related
- Consult technical service bulletins or forums for known weak points of your model
- Consider professional diagnostics with more advanced equipment
Intermittent faults are difficult to trace. They are not always present during testing. Try to reproduce the problem under the same conditions as when the fault occurred before. Each time, note the exact situation:
- road speed
- engine temperature
- load
This information helps when identifying the pattern.
Prevention is better than diagnosis
Preventive maintenance reduces the risk of fault codes. Regularly check fluid levels, filters, spark plug leads and belts. This way you avoid many common problems. Replace wear parts in good time. Think of the battery, sensors and seals. This contributes to reliability.
Many fault codes arise from aged or contaminated components. These no longer operate within specifications. A dirty air filter can, for example, lead to a fault code relating to the air–fuel mixture. A flat battery sometimes causes strange codes because control units receive too little voltage. Maintain your vehicle properly. Spot irregularities early. This way small problems do not grow into major repairs.
On the Carnews website you will find much more information. For example, about common car problems, maintenance advice and tips for recognising issues. We cover various makes and models. Discover the background of technical faults and learn how to carry out simple checks yourself.
Frequently asked questions
Never ignore red warning lights (such as oil pressure, coolant temperature or braking system), because they mean you must stop immediately. Serious new noises (loud ticking, scraping, whining) or strong vibrations while driving also require urgent attention. You should also never ignore thick smoke or a strong smell of burning, oil or coolant around the car. Finally, poorly responding brakes, a heavy or “floating” steering feel and sudden loss of power are all signals that you should visit a garage immediately.
Essential for the beginner home mechanic are a simple OBD2 diagnostic tool to read fault codes and view live data, and a reliable digital multimeter for measuring voltage, resistance and continuity. In addition, you need a set of basic hand tools, including a torque wrench, and a good jack with axle stands so that you can work safely under the car. A work light/inspection lamp and tools for checking coolant and oil levels (dipstick, filling sets) help you quickly check basic conditions. Finally, a workshop manual or reliable online documentation is indispensable for carrying out test and repair procedures correctly.
Always consult your car’s manual: it explains what each light means and how urgent it is. In general, the rule is: red = stop immediately and (if safe) switch off the engine, amber/yellow = have it checked as soon as possible, and green/blue = informative (for example, lighting). Pay attention to combinations of symptoms (strange noise, smoke, loss of power, smell) to decide whether you must stop at once or can still drive carefully to a garage. If in doubt, especially with red or flashing lights, do not continue driving but call the emergency services or roadside assistance.
Common engine problems include ignition or injection issues, clogged filters, turbo problems and defective sensors. You can recognise them from noises such as uneven running, rattling, pinking, a whooshing sound (turbo) or loud blowing at the exhaust. Loss of performance shows as poor response when accelerating, hesitation under load, increased fuel consumption and sometimes smoke (black, white or blue) from the exhaust. Rough idling, difficult starting and warning lights (engine/emissions light) are also clear signs.
It is wise to call in a professional in the case of a vague complaint as soon as the complaint returns, gets worse or affects safety or reliability (such as starting problems, loss of power or warning lights). If, after checking obvious causes (battery, fuses, fluids, visible leaks), you do not find a clear cause, professional diagnosis is necessary. When the complaint only occurs under certain conditions and is difficult to reproduce, a specialist can identify patterns using targeted measurements and fault code reading. In addition, for commercial vehicles or if you depend on the vehicle for work, it is advisable to consult a professional sooner to prevent breakdowns and costly consequential damage.