CAN Bus Explained: Understanding Modern Car Communication Networks

When you have a problem with your car, a part could be to blame. Yet, we have heard of many instances, where customers have wasted money having parts changed but the problem persists. A reason for this tends to be incorrect, or incomplete diagnosis, resulting in components being replaced blindly, much to the annoyance of the car owner. In reality, the car's data communication network might have been at fault.

As terminology between mechanical components and computers is so different, even professional technicians can struggle. So, because car networks are so misunderstood, we hope this introduction blog on the topic is useful by car owners and professional technicians alike.

Why do modern cars need communication networks?

As modern cars are fitted with so many electrical comfort and safety features, the number of computer control units has ballooned. This meant that car wiring harnesses became larger, heavier and more expensive for the car manufacturers. Garages also found them extremely difficult to fault diagnose. The need for computers not just to communicate with the components they control but also with each other made the wiring even more complicated.

The simplest solution was to link the various computers together (most commonly with a wire) to form a network - known also as a 'bus'. This would not carry electricity to power the components but data communication signals. Each computer would receive data signals relevant to itself and ignore the others, before passing them on.

What does CAN bus stand for?

CAN bus stands for Controller Area Network. It is a communication protocol developed to allow multiple electronic control units (ECUs) within a vehicle to exchange information quickly and reliably without needing dedicated wiring between every component.

Rather than each computer operating independently, CAN bus enables systems such as the engine, transmission, braking, steering and safety features to communicate with one another across a shared network. This reduces the amount of wiring required, helping manufacturers build lighter, more reliable and easier-to-maintain vehicles.

Today, CAN bus technology is found in almost every modern petrol, diesel, hybrid and electric vehicle, making it one of the most important communication systems in automotive engineering.

What is CAN bus and how does it work?

The data language (protocol) also varies between networks (buses). The speed at which data is transferred also differs. Therefore, a modern car can possess multiple

buses, each running different protocols and data speeds. One bus may be for the engine and gearbox, another for the interior comfort features, another for the multimedia equipment and so on.

Various protocols (i.e. data languages) are employed, the majority of which use copper wires. The most common is LIN (Local Interconnected Network), which uses a single wire to receive data. Should the wire break, communication fails and the components on that particular network will not work. Most features using the LIN protocol, therefore, have a manual override switch. Typical examples include automatic headlamps, or windscreen wipers.

CAN (Control Area Network) is a protocol that tends to be used for the engine, gearbox and safety features. It uses a pair of wires that are twisted together (as pictured), meaning that control units can both receive and transmit data. Should one wire develop a fault, the other one will take over. CAN bus, therefore, is extremely dependable. Most problems tend to be caused by modifications being performed to the vehicle, such as electrical accessories being fitted.

Therefore, if you are told that your car has CAN bus, the correct terminology is that you have a networked vehicle that uses CAN bus protocol for one, or more, of its buses.

CAN bus was invented by Bosch but some car manufacturers use their own twin-wire protocols. Typical examples include Ford's SCP (Standard Corporate Protocol) and General Motors-era Vauxhalls using GM_CAN. Another alternative to CAN bus is FlexRay.

MOST (Media Oriented Systems Transport) is another protocol. Instead of using copper wires, it employs fibre-optic cables. The most common application is in multimedia systems, such as satellite navigation units. Diagnosing MOST problems is not easy but a kinked fibre optic cable will cause communication issues. Byteflight is a fibre-optics-based data bus, used commonly by BMW, for certain safety critical items, including airbags.

What are the different types of vehicle communication networks?

Modern vehicles rarely rely on a single communication network. Instead, different systems use different protocols depending on the speed and reliability required.

LIN (Local Interconnect Network)

LIN is commonly used for lower-speed electrical features, including:

• Automatic headlights
• Electric mirrors
• Rain-sensing wipers
• Climate control components

Because LIN uses a single communication wire, it is inexpensive but less fault tolerant than CAN.

CAN (Controller Area Network)

CAN is the most common automotive communication protocol and is typically responsible for critical systems such as:

• Engine management
• Automatic transmission
• ABS
• Airbags
• Stability control

Its twin-wire design provides excellent reliability, even if electrical interference occurs.

MOST (Media Oriented Systems Transport)

MOST uses fibre-optic cables rather than copper wiring and is mainly found in:

• Satellite navigation
• Premium audio systems
• Multimedia displays
• Entertainment systems

Because data travels using light rather than electricity, damage to a fibre-optic cable can interrupt communications.

FlexRay

FlexRay was developed for extremely fast communication between safety-critical systems and has been used on a number of premium vehicles.

It supports technologies including:

• Active suspension
• Advanced driver assistance systems (ADAS)
• High-speed chassis control

Although newer Ethernet-based vehicle networks are becoming more common, FlexRay is still found on many existing vehicles.

Manufacturer-specific communication systems

Some manufacturers have developed their own communication protocols alongside CAN, including:

• Ford SCP (Standard Corporate Protocol)
• GM_CAN (used on certain Vauxhall and General Motors models)
• Other proprietary systems designed for specific vehicle platforms

These networks all perform similar functions but require manufacturer-specific diagnostic information when faults occur.

What happens when a car communication network fails?

When a communication network fails, symptoms vary from nothing at all to the engine failing to start. The engine may even continue to run. We have heard of some Battery Electric Vehicles not coming out of their 'Ready' mode, meaning that the vehicle cannot be locked. Most commonly, components on the affected network will cease to function, which provides a useful diagnostic clue. Therefore, if many components stop working at the same time, the network might be responsible.

However, low-voltage situations cause communication faults, too. Therefore, ensure that your 12v battery has been tested for optimum state of charge and state of health.

What are the signs of a CAN bus problem?

Communication network faults often produce unusual symptoms because multiple vehicle systems depend on shared information.

Common warning signs include:

• Multiple dashboard warning lights appearing simultaneously.
• Several electrical systems failing at once.
• Intermittent starting problems.
• Engine refusing to start.
• Windows, lights or wipers behaving unpredictably.
• Loss of communication during diagnostic testing.
• Safety systems reporting faults despite no obvious mechanical problem.

Because these symptoms can resemble component failures, proper diagnosis is essential before replacing any parts.

Can a weak battery cause CAN bus faults?

Yes. A weak or failing 12-volt battery is one of the most common causes of communication faults in modern vehicles.

Electronic control units require a stable voltage to communicate correctly. If battery voltage drops too low, computers may begin shutting down, resetting or losing communication with one another.

Symptoms may include:

• Random warning lights.
• Fault codes affecting multiple systems.
• Electrical features operating intermittently.
• Failure to enter or leave "Ready" mode on hybrid and electric vehicles.
• Difficulty starting the engine.

Before investigating complex wiring faults, many technicians will first test the battery's condition and charging system.

How are CAN bus and network faults diagnosed?

Detailed checks are not really possible for a typical car owner. One reason is that you will need detailed information about the appropriate network(s) fitted to your car, including its layouts, protocols used and data speeds. A proficient garage should have access to this data and be able to identify whether the appropriate network is capable of transmitting messages, whether any signals are being transmitted and, if so, what data is being transmitted.

This diagnostic work takes time and is more involved than a straight-forward check for fault codes, with a basic diagnostic tool. Some car owners think that paying for diagnostic time is a waste of money. Yet, very often, it is preferable to throwing cash away, by replacing perfectly serviceable parts.

Can you repair a CAN bus fault yourself?

Simple checks can sometimes be carried out at home, but diagnosing CAN bus faults is generally beyond the scope of most DIY repairs.

Owners can safely:

• Check battery voltage.
• Inspect obvious wiring damage.
• Ensure recent accessories have been installed correctly.
• Look for loose electrical connectors where accessible.

However, accurately locating communication faults usually requires:

• Manufacturer wiring diagrams.
• Professional diagnostic equipment.
• Oscilloscopes.
• Network analysis software.

Attempting repairs without the correct information can easily introduce additional faults or damage sensitive electronic systems.

Why is proper diagnosis important?

Replacing parts simply because a fault code mentions them can become an expensive mistake.

For example, a communication fault may cause several control units to report errors, even though none of those components are actually faulty. Replacing sensors, modules or actuators without confirming the underlying cause can quickly lead to hundreds—or even thousands—of pounds in unnecessary repair costs.

Professional diagnosis focuses on identifying the root cause before any parts are replaced. This may involve checking wiring integrity, network communication, battery voltage and data signals between control units.

Although diagnostic testing takes time and may involve labour charges, it is often far more cost-effective than replacing perfectly serviceable components through trial and error.

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