CAN bus car security — how it actually works and what to check before you buy

Keyless entry made cars easier to steal. A relay attack takes seconds, needs no key, and leaves no damage. The response from the aftermarket has been the CAN bus immobiliser: a small module that sits on the car’s own data network and refuses to let the engine start until it recognises the driver.

This article is for the buyer who has already worked out that they need something beyond the factory system, and now wants to understand what they’re actually buying — how the technology works, where the engineering quality shows up, and which questions to ask an installer before anyone touches the car.

What the CAN bus is, in plain English

Think of the CAN bus as an internal group chat. When you press the start button, the body control module announces it. The immobiliser confirms the key is authorised. The engine ECU sees both messages and fuels the engine. Nothing physical needs to move between those modules — it’s all conversation.

That’s the opportunity and the risk in one sentence. A well-designed security module joins the conversation to add a second authorisation step that a thief can’t supply. A badly-designed one talks over everything else and confuses the modules that were there first.

Where the module sits The CAN bus links every control unit in the car. A security module joins that network and adds one more condition before the engine is allowed to start.

How a CAN bus immobiliser stops a keyless theft

A relay attack works by extending your key’s signal from inside the house to the car on the drive. The car believes the key is present, unlocks, and starts. Nothing is forced and nothing is broken. The factory immobiliser is satisfied because, as far as it can tell, the correct key answered.

A CAN bus immobiliser breaks that chain by adding an authorisation step the relayed key can’t complete.

“A security module has to be a good citizen on the network. It listens, it adds one condition, and when the car is asleep it stays quiet. That’s the whole engineering brief, and it’s where most cheap modules fall down.” — Victor, founder and design engineer, LockCar

Three things to check before you buy any CAN system

CAN bus immobilisers are not all the same, and the differences don’t show up in the first week. These are the three questions worth asking any installer, about any brand — ours included.

What you get as standard vs what CAN-IMMO adds

This compares a modern keyless vehicle with its factory security only — the situation most owners are in — against the same vehicle with CAN-IMMO fitted.

Decision guide: which setup fits your vehicle

CAN-IMMO is the right answer for most modern cars, but not for all of them. Here’s the honest routing.

If your vehicle isn’t obviously in one of those rows, send the make, model and year and we’ll tell you which of the two applies. It takes one message and costs nothing.

The LockCar range — CAN-IMMO and the rest

LockCar makes nine products across three categories. These are the ones most people are choosing between, and who each is for.

For high-value vehicles where detection range matters, the IS357 adds a 5m proximity range and smartwatch or smart ring support on top of the standalone platform.

The bottom line

Keyless theft succeeds because the car asks one question and the thief has learned how to answer it. Everything else — the trackers, the wheel locks, the security lighting — deals with what happens after that answer works.

A CAN bus immobiliser changes the question. Done well, it costs you nothing in daily use: you carry a tag, you walk up, the car starts. Done badly, it costs you warning lights and flat batteries. The difference is entirely in whether the firmware was written for your vehicle and whether the module knows how to stay quiet when the car is asleep.

That’s the standard CAN-IMMO was built to. At £149 for the unit and from £320 fitted, with installation by the person who designed it, it’s the version of this technology we’d want on our own cars.