Why are my LED lighting units flashing? Causes and solutions for CAN-bus interference

Sooner or later, many mechanics and vehicle manufacturers will encounter CAN-bus interference with LED lighting. However, in most cases, the CAN network itself is not faulty. The cause usually lies with the vehicle’s electronic lamp monitoring system, which may interpret the lower current draw of LED lighting as a fault.

In this article, we explain:

• What interferences can occur?
• How to check the cause
• when a resistance, control box or other solution is required

Why do LED lighting systems display a CAN-bus error message?

Modern trucks, vans and cars use a CAN-bus: a data network via which the vehicle’s control units communicate with one another. For lighting, lamp monitoring is particularly important. The control unit that manages the lighting measures the current flowing through each light function and compares this with the power consumption of an incandescent bulb.

A ‘CAN-bus interference’ with LED lighting is therefore almost always a message from the lamp monitoring system and not interference in the network itself. An LED consumes a fraction of the current of an incandescent bulb, so the control unit concludes that the light is faulty. This message is sent via the CAN-bus to the dashboard, but the cause lies in the current measurement on the lighting circuit. That is why a resistance, a control box or an adjusted vehicle configuration will resolve the problem: the lamp monitoring system either detects the expected load again or ceases to monitor it.

You can recognise a genuine network fault by other symptoms, such as multiple systems failing simultaneously or error messages unrelated to the lighting. Even vehicles without a CAN-bus can react to LED lighting: a flasher relay that operates under load will then start flashing too quickly. Would you like to know how the CAN-bus works and what ‘canbus proof’ lighting entails? Then read ‘What is canbus proof LED lighting?’.

Important to note: what is commonly referred to in practice as a “CAN-bus interference” is usually not a problem within the CAN network itself. It almost always relates to light monitoring (load detection): the vehicle measures the current draw from a light to check whether it is still working. An LED consumes so little power that the system incorrectly interprets this as a fault. Those who understand this distinction will look for the solution in the right place. You can read more about this in our article ‘What is CAN-bus-proof LED lighting?’.

Would you rather go straight to the solutions? Then take a look at the available CAN-bus solutions.

---

Lighting for semi-trailers and heavy goods vehicles

Since 2005, TRALERT® has specialised in ECE-approved, shock- and water-resistant LED lighting for semi-trailers, trailers and heavy goods vehicles.

See our lighting for heavy transport →

---

Why do LED lighting systems cause CAN-bus interference?

LED lighting uses significantly less power than a conventional filament bulb. When the lamp monitoring system anticipates a higher load, the vehicle may incorrectly identify the LED light as faulty. Depending on the vehicle and the light function, this can lead to an error message, hyperflashing, pulsating lighting or the output being switched off.

A second possible cause is short diagnostic pulses from the light control module. With an incandescent bulb, these are usually not visible, but an energy-efficient LED may flash briefly or remain dimly lit as a result. Not every symptom is therefore caused by exactly the same problem.

Identifying CAN-bus interference by its symptoms

Symptom	Check first	Possible solution
Symptom:The light is working, but the dashboard is displaying an error message	Check first: Plug, connection and correct light function	Possible solution: Suitable load resistor or control box
Symptom: The indicator is flashing too quickly	First, check whetherall indicators are working and correctly connected	Possible solution: A resistor or control box suitable for the load
Symptom: The LED pulses or flashes whilst switched off	First, check whetherthe light control module is emitting diagnostic pulses	Possible solution: A suitable control box or module
Symptom: The lighting works for a short period and then fails	First, checkthe vehicle’s voltage, mass, wiring and fault code	Possible solutions: control box, vehicle configuration or circuit repair
Symptom: Severallight functions are operating erratically	Check first: Connector, mass, pin configuration and wiring harness	Possible solution: First rectify the installation, then retest it

Always start by checking the plug connection, mass and cabling. A poor connection, corrosion or a broken cable is not a CAN-bus problem and will not be resolved simply by fitting a resistance or control box. You can read more about this in our guide to interference in lorry and semi-trailer lighting.

LED lighting uses much less power than traditional bulbs. As a result, the CAN-bus system sometimes thinks a light is faulty. This leads to error messages, flashing lighting or even failure.

Solutions for CAN-bus issues

The appropriate solution depends on the system voltage, the specific light function and the way in which the vehicle monitors the lighting. You should therefore first check whether the interference is actually caused by the lower load of LED lighting.

• CAN-bus-compatible LED lighting: lighting whose electrical features are better suited to electronic lamp monitoring. Compatibility may vary depending on the vehicle and application.
• Load resistance: increases the load on a single light function, so that it more closely matches that of a conventional light. The resistance generates heat and must be correctly sized and fitted.
• Control box: a centralised solution for one or more lighting functions. Depending on the model, this is connected via a plug-in connection or integrated directly into the wiring harness.
• Vehicle configuration: on some vehicles, an authorised workshop can configure the lamp monitoring system or semi-trailer module via software for LED lighting.
• Repairing cabling or connections: necessary when the interference is caused by corrosion, a poor mass connection, incorrect pin configuration or voltage loss.

When should you choose a resistance and when a control box?

A separate load resistor is particularly useful when a single specific 12V light function causes an error message or an excessively fast flashing frequency. For multiple light functions, a complete 13-PIN connection or a 24V lorry and semi-trailer system, a suitable control box is usually a more straightforward solution.

The LR12 , for example, is a 12V load resistor for a separate circuit. It simulates an additional load and, due to the heat generated, must be mounted on a suitable metal surface. For complete connections, there are 12V control boxes with a 13-PIN connection and a 24V control box that is integrated directly into the truck or semi-trailer wiring.

View the available CAN-bus solutions or, if in doubt, first identify which light function and load are causing the interference.

Flashing lighting often indicates a mismatch between LEDs and CAN-bus detection requirements.

Preventing CAN-bus issues with LED lighting

When replacing or performing the first mounting of LED lighting, you can prevent much interference by taking the electrical connection and vehicle monitoring into account when selecting your products.

• Check whether the system is 12V or 24V.
• Determine which light functions are monitored electronically.
• Choose lighting with the correct ECE approval for the relevant light function.
• Check that the lighting is electrically compatible with the vehicle’s lamp monitoring system.
• Check connectors, earth connections, pin configurations and the wiring harness before commissioning.
• For complete vehicle body builds, be sure to include lighting, cabling and any control boxes in the design from the outset.

CAN-bus-compatible and ECE R10: what is the difference?

ECE R11 concerns electromagnetic compatibility (EMC). The lighting must not cause interference with other electronic equipment, such as the radio or vehicle systems, nor must it itself be affected by such interference. The standard does not specify how your vehicle’s lamp monitoring system reacts to the light.

An R10-approved LED light can therefore still trigger a ‘light faulty’ warning. Conversely, a resistance or control box will not resolve radio interference caused by LED lighting. When purchasing, therefore, check both: R10 approval for EMC, and CAN-bus compatibility or the correct load for the lamp monitoring system.

By coordinating lighting, cabling and electronic monitoring right from the design phase, you can prevent repair work and interference after handover.

CAN-bus interference on a trailer: ‘light faulty’ message

A common scenario is that the trailer’s indicator works normally once or twice and then starts flashing more rapidly, fails, or triggers a ‘light faulty’ warning on the dashboard. The LED light itself is usually not faulty. The light monitoring system on the towing vehicle detects insufficient current on that circuit and therefore identifies the light as faulty.

First, rule out a cabling fault. If the lighting does not work at all or only intermittently, the cause usually lies with the connector, the mass or the cabling, rather than the bulb monitoring system. Check the 13-PIN or 7-PIN connector for corrosion and the earth connection for voltage drop. You can find the full step-by-step guide in ‘Trailer lighting not working: causes and solutions’.

Is the cabling in good condition? Here’s how to determine whether lamp monitoring is the cause:

• The warning only appears for functions using LED lighting, for example, only for the indicator. This indicates that the load on that circuit is too low.
• The warning message disappears if you temporarily fit an incandescent bulb back in. This is a strong indication that the lamp monitoring system does not recognise the LED.
• If the tow bar has been retrofitted, check that the trailer module is correctly installed and configured. Many modules have their own lamp monitoring system or a setting for LED lighting.

Is a low load the cause? If only one function is triggering an error message, a resistance on that circuit is often sufficient. If several functions are triggering an error message, or if you wish to avoid heat build-up caused by resistors, an LED control box for the 13-PIN connection is the cleaner solution.

By cooperating with specialists as early as the design phase, you save time, money and interference afterwards.

Frequently asked questions about CAN-bus interference

Why do the LED lights on my trailer flash or flicker after replacing the bulbs?

This is usually caused by the headlight monitoring system of the towing vehicle, which measures the current flowing through each light function. An LED consumes much less power than an incandescent light, causing the vehicle to think the light is faulty. The result is rapid flashing, flickering or an error message. A resistance, a control box or CAN-bus-compatible lighting resolves this issue.

Why does my truck display ‘light faulty’ when the LED is clearly lit?

The light monitoring system detects too little load on that circuit. The light is therefore working, but the vehicle does not recognise it as a functioning light. This is not an interference with the CAN-bus itself, but an incorrect current reading. With a suitable resistance or control box, the vehicle will once again detect the expected load.

How do I know if it’s CAN-bus interference or a faulty light?

Temporarily fit an incandescent bulb back into the function that is triggering the warning. If the warning disappears, the lamp monitoring system does not recognise the LED and the light itself is in good working order. If the warning persists, or if the lighting does not work at all, this does not automatically indicate CAN-bus interference. In that case, first check that you have connected the trailer lighting correctly and that the connector, mass and wiring are in good working order.

Why is my LED indicator flashing too quickly?

Rapid flashing, also known as hyperflashing, is the way in which the vehicle indicates a faulty indicator. The lamp monitoring system detects insufficient current and doubles the flashing frequency as a warning. A load resistor on the indicator circuit or a control box will resolve this.

Why do LED lights flash or glow when they are switched off?

Many vehicles send short truck pulses or a small residual current through the lighting circuits to check whether a light is present. An incandescent bulb does not visibly respond to this, but an LED does. As a result, it flashes or glows faintly. A control box or LED lighting system suitable for bulb monitoring prevents this.

Do I always need resistance with LED lighting?

No. A resistance is only required if the vehicle has lamp monitoring and the LED provides insufficient load. Many vehicles do not recognise LED lighting, and CAN-bus-compatible LED lights often have the required load built in. In the event of interference affecting multiple functions, a control box is usually the better choice. You can read more about this on the page about resistance.

Is a load resistor getting hot?

Yes. A load resistor converts the excess current into heat and can become very hot. You should therefore mount it on a metal surface, away from cables, plastic and flammable materials. If you wish to avoid heat build-up, a control box is an alternative.

Does this also apply to 24V lorries?

Yes. 24V lorries and tractor units often have lamp monitoring systems and may report LED lighting as faulty. In this case, use a resistance or control box suitable for 24V. A 12V resistance is not suitable for a 24V system.

Can a vehicle be software-upgraded for LED lighting?

This is the case for some vehicles. Using diagnostic equipment, the lamp monitoring can be adjusted per function or set to LED mode, usually via the dealer or a specialist workshop. This is not possible with every brand or model. Please bear in mind that the fault indicator for indicators is a legal requirement, so do not simply switch it off.

Is CAN-bus-compatible lighting the same as ECE R10?

No. ECE R10 relates to electromagnetic compatibility: the lighting must not interfere with other electronic equipment, such as the radio. CAN-bus compatibility relates to how the vehicle’s light monitoring system responds to the light. A light may comply with ECE R10 and still trigger a ‘light faulty’ warning.

From individual CAN-bus solutions to complete lighting systems

In the case of an isolated fault message, a standard solution, such as a load resistor or control box, is often sufficient. Take a look at our CAN-bus solutions for vehicle lighting.

When it comes to complete vehicle bodies, trailers or bespoke cabling, there are more factors to consider. These include system voltage, light functions, wiring harness configuration, connectors and the vehicle’s electronic monitoring systems. TRALERT® can coordinate these components right from the drawing board and supply them as a complete lighting and cabling solution.

View our 24V semi-trailer lighting and plug-&-play cabling, or get in touch for support with a complete vehicle or cabling project.