Lighting and electrical switches are a major problem area in cars, particularly faulty number plate lights, incorrect dipped beam alignment and colour deviations in indicators. These problems regularly lead to failure during the MOT test. Many owners underestimate the impact of electrical faults, while a small defect can have major consequences for road safety and the test result. Find out which problems you can expect and how to tackle them effectively.
Number plate lighting: a source of test failures
Faulty number plate lighting affects 14% of cars, making it one of the most common reasons for MOT failure. This problem may seem minor, but it is a serious fault that can directly result in a failed MOT test.
The cause is often a simple part: a blown bulb or an oxidised bulb holder. However, in many models, including the Peugeot Expert, an electrical issue can also be involved. A faulty relay or poor earth connection can cause the number plate lights to fail completely.
First check the following parts:
- Replace the bulb and check that the holder is clean and dry
- Inspect the wiring for damage caused by rust or moisture
- Test the earth connection at the rear lights
- Check the fuse that controls the rear lights
If the problem persists, a faulty relay under the dashboard may be the cause. This is common in vans and older cars that are used intensively.
Dipped beam alignment prevents dangerous situations
Incorrect dipped beam alignment causes glare for oncoming traffic or insufficient visibility for the driver. This problem often arises after replacing a bulb, driving with a heavy load or following a minor collision.
Dipped beam alignment must be carried out according to the manufacturer’s specifications. For most cars, the light beam should fall about 10 centimetres below the horizontal line at a distance of 10 metres. Beams set too high dazzle other road users; beams set too low reduce visibility.
Here is how to adjust the dipped beam correctly:
- Park the car on level ground with the nose facing a wall, 5 metres away
- Make sure the tank is half full and the tyre pressures are correct
- Mark the centre line of each headlight on the wall with tape
- Reverse to a distance of 10 metres
- Turn the adjustment screw behind the headlight until the beam sits 10 centimetres below the mark
On modern cars with xenon or LED lighting, professional adjustment is often necessary. These systems have automatic levelling that requires complex diagnosis when faults occur.
Windscreen wipers that do not provide enough visibility
Windscreen wipers that work poorly pose a direct safety risk and lead to MOT failure. The problem shows itself as streaks on the windscreen, incomplete wiping movements or wipers that stop halfway.
The cause is often worn wiper blades, but electrical faults are also common. A faulty wiper motor, a broken switch or a problem in the wiring can cause the wipers to fail completely or operate at irregular speed.
Common causes of wiper problems:
- Worn wiper blades leaving rubber residue on the windscreen
- Faulty wiper motor that no longer responds
- Broken switch on the steering column
- Damaged wiring due to moisture ingress
- Empty washer fluid reservoir that no longer sprays
Replace wiper blades at least once a year. If you have problems, check the wiper motor fuse first before interfering with the electronics. Many faults can be fixed easily by fitting a new switch or cleaning the contacts.
Indicators with colour deviations
Indicators must emit orange light according to the law. Colour deviations, such as yellow or white light, lead directly to MOT failure. This problem often arises from using the wrong bulbs or LED conversions without type approval.
Another common problem is indicators flashing too fast or too slowly. This usually points to a faulty bulb or incorrect resistance in the circuit. With LED lighting, a missing load resistor can cause the flashing rate to be incorrect.
Pay attention to the following points when dealing with indicator problems:
- Only use bulbs with the correct specifications and orange colour
- When converting to LED, always install approved units with the correct resistance
- Check that all indicators flash at the same rate
- Test both the front and rear lights to make sure they work correctly
Fast flashing often indicates a blown bulb on one side. Always replace both bulbs at the same time for even brightness and colour. Slow flashing can point to a faulty relay or the wrong bulb with too low a wattage.
‘ This is a low-cost repair you can carry out yourself. ’
Faults caused by defective relays and switches
A purple relay under the dashboard is the cause of widespread lighting problems in many cars. This relay controls the power supply to various lighting systems. Internal wear or overheating can cause the relay to fail, making lights switch on and off automatically or fail completely.
The solution is often simple: remove the relay, clean the contacts and refit it. In many cases, this solves the problem immediately. If it does not, the relay needs replacing. This is a low-cost repair you can carry out yourself.
Another common cause is a faulty light switch on the steering wheel. The dipped beam and main beam switch can break over time, causing the headlights to stop responding. In that case, replace the entire stalk, as the switch and indicator are often integrated as one unit.
CAN bus problems lead to complex faults
With widespread electrical faults where several systems fail at the same time, the cause is often the CAN bus. The CAN bus (Controller Area Network) is a communication system that allows electronic components in the car to communicate with each other. If one module is faulty, communication with other modules can fail.
This leads to problems with lighting, electric windows, central locking and other systems. Diagnosis requires a systematic approach in which each module is tested individually. Also check the two resistors in the CAN bus circuit, as they are often faulty in these cases.
Steps for CAN bus diagnosis:
- Check all fuses first before interfering with the CAN bus
- Find a wiring diagram for your specific model to verify connections
- Test each module individually by temporarily removing it
CAN bus problems often occur around 160,000 kilometres as a result of wear and corrosion in modules. Preventive maintenance by regularly checking fuses and contacts can prevent many problems.
Preventive maintenance prevents lighting problems
Regular checks of lighting and electrical systems prevent surprises during the MOT test. Test all lights at least once a month, including number plate lights, brake lights and indicators. Replace faulty bulbs immediately to avoid fines and test issues.
Clean the contacts of bulbs and fuses twice a year. Corrosion and moisture are the main causes of electrical faults. Use contact spray to remove oxidation and improve the connection.
If widespread or complex faults occur, have the electronics read professionally. A diagnostic computer can read fault codes from the CAN bus that point directly to the cause. This saves time and prevents unnecessary replacement of parts.
On the Carnews website you will find much more information about electrical problems, maintenance tips and model-specific faults by car make. Discover how to keep your car in top condition and avoid test failures by regularly visiting our extensive knowledge base.
Frequently asked questions
How can I identify common problems with car lighting or electrical switches myself?
Start by checking all fuses for the lights and switches, and replace any blown ones. Next, find the lighting relay under the dashboard, which is often purple, remove it, clean the contacts and test with a new relay if the problem remains. If the lights sometimes work and sometimes do not, or dipped beam/main beam and indicators do not respond properly, test the light/steering stalk by moving it or applying slight pressure while operating it and replace it if in doubt. If several electrical functions continue to fail at the same time, have the CAN bus modules and resistors tested against the wiring diagram, as that is where the fault is likely to be.
What safety risks and performance impacts do faulty lights or switches have, especially in motorsport or at high speed?
Faulty lights or switches reduce the visibility and recognisability of the vehicle, greatly increasing the risk of collisions at high speed, especially in motorsport environments with short reaction times. Headlights or brake lights failing at speed can cause the driver to become disorientated and following drivers to react unexpectedly. Electrical faults in switches can also affect other systems (such as CAN-bus-dependent functions), which can cause sudden and unpredictable vehicle responses. All this often forces the driver to brake earlier and turn in more cautiously, which negatively affects lap times and overall performance.
How can I proactively prevent problems with my car’s lights and electrical switches, and which checks are essential?
You can proactively prevent problems with lights and switches by regularly testing all lighting (dipped beam, main beam, brake lights, indicators) and having any irregularities checked straight away. Periodically inspect the fuses and relays (especially the relevant light relay under the dashboard) for corrosion, heat marks and loose contacts, and replace any doubtful parts. If there are unexplained or multiple simultaneous faults, have the CAN bus and associated modules read professionally and the wiring checked against the diagram. Finally, test the light/steering switch for play or failure (for example, an indicator or main beam that does not stay engaged properly) and replace the complete stalk when worn.
Are there specific upgrades or modifications for lights or switches that improve performance or reliability for car enthusiasts?
Yes, there are several upgrades that can improve both performance and reliability, such as fitting high-quality LED or xenon headlights with a better beam pattern and higher efficiency. Replacing standard relays with quality relays and using waterproof, corrosion-resistant connectors and cables also increases reliability. For switches, many enthusiasts choose heavy-duty or motorsport switches with a higher current rating and better mechanical quality. In addition, reworking the wiring with separate power feeds and fuse boxes can reduce voltage drop and extend the life of lights and switches.
Which lighting or switch problems can I best fix myself, and when is it advisable to call in a professional?
Simple jobs such as replacing bulbs, checking or replacing fuses, and cleaning or easily swapping out an accessible relay can usually be done yourself. You can also often fix a clearly visible loose connector or corrosion on a connector contact by cleaning and reconnecting it. It is wise to call in a professional for complex or widespread electrical faults (several systems at once), suspected CAN-bus or module problems, and when switches in the steering column or dashboard need replacing. Also get an expert involved if you cannot read wiring diagrams, need special diagnostic equipment or are unsure about safety.
