How do you test a CAN bus?

The quickest meaningful test needs only a multimeter. With the ignition off and the car left to go to sleep, measure the resistance between the CAN High and CAN Low pins at the diagnostic socket — pins 6 and 14 on a standard connector. A healthy high-speed network reads around sixty ohms, because two one-hundred-and-twenty-ohm terminating resistors sit at opposite ends of the bus and appear in parallel. Roughly one hundred and twenty ohms means one terminator or one branch has dropped off. Close to zero means the two wires are shorted together. A very high reading means the bus is broken. From there an oscilloscope shows the waveform and identifies which module is misbehaving.

Can one faulty module take out the whole car?

Yes, and it is the single most important thing to understand about network faults. Every module shares the same pair of wires, so a module that fails with an internal short pulls those wires together and silences every other module on the bus. The dashboard lights up like a Christmas tree and the car may not start, which looks like a catastrophic and expensive failure. In reality one part is broken and the rest are simply unable to speak. That is why disconnecting modules one at a time until the network recovers is the standard approach.

Can a dashcam or tracker cause a CAN bus fault?

Yes, and badly installed accessories are one of the more common causes in practice. Anything spliced into the bus wiring — trackers, alarms, parking sensors, towbar electrics, some dashcam installations — introduces a new connection into a circuit designed to very specific electrical characteristics. A poorly made splice, an added length of untwisted wire, or a device that transmits when it should not can all upset the network. If the trouble started soon after something was fitted, say so, because it saves a great deal of diagnostic time.

Will U0001 fail an MOT?

Almost certainly, and probably several times over on one certificate. A network failure typically lights the ABS, airbag, stability control, tyre pressure and engine management lamps, and every one of those is a separate major defect: 85,196 recorded failures for the brake system warning device, 84,147 for the airbag lamp, 35,322 for stability control, 97,515 for tyre pressure monitoring on cars first used from 1 January 2012, and 371,859 for the engine management lamp. It is one of the few faults that can produce a failure certificate with a whole list of major defects from a single underlying cause.

Network code · standard

U0001The car's main communication network has failed

Standard definition: High Speed CAN Communication Bus

Last updated 27 July 2026

Quick answer

The high-speed data bus that every major control module uses to talk to the others is not working properly. It is rarely one module at fault; it is usually the wiring they all share, or one module dragging the whole network down.

How the car works this out

Modern cars connect their modules with a twisted pair of wires called CAN High and CAN Low, carrying messages that every module can hear. Each module counts the errors it sees, and when the count passes a threshold it declares the bus unusable and stops transmitting — a state called bus-off. U0001 is set when that happens, or when the electrical characteristics of the bus itself go outside what the standard allows. A single module failing internally can short the pair together and silence the entire network, which is why a fault that looks catastrophic often comes down to one part.

What you might notice

Multiple warning lamps illuminated at once, sometimes the entire dashboard
A scan tool unable to communicate with any module, or with several
The car refusing to start, or starting and running erratically
Gauges dead or reading nonsense
Central locking, lights and wipers behaving unpredictably
Everything working normally after a restart, then failing again

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A single failed module shorting the bus and silencing everything elseCommon
How to check: Disconnect modules one at a time and see whether the network recovers. When it does, the last one removed is the culprit. It is methodical rather than clever, and it is how these are actually found.
2Damaged bus wiring — the pair shorted together, to earth or to twelve voltsCommon
How to check: With the ignition off, measure the resistance between the CAN High and CAN Low pins at the diagnostic socket. A healthy network reads about sixty ohms, because two one-hundred-and-twenty-ohm terminating resistors sit in parallel. Around one hundred and twenty means one terminator or one branch has been lost; near zero means the wires are shorted together; very high means the bus is broken.
3Water in a connector, particularly at the bulkhead, under a seat or in a doorCommon
4Aftermarket accessories spliced into the bus — trackers, dashcams, alarms, stereos or towbar electricsCommon
How to check: Ask what has been fitted to the car and by whom. Badly installed accessories tapped into the network are one of the most common causes and among the easiest to overlook.
5Low system voltage from a failing battery or alternator, which makes modules drop off the networkPossible
How to check: Have the battery load tested first. It is cheap and it eliminates a genuine and common cause of what looks like a catastrophic electrical fault.
6Corroded earth points, which the whole network depends on for its referencePossible

What to do about it

1Have the battery and charging system checked before anything else. A car with a dashboard full of lamps and a tired battery is describing a battery problem more often than a network one.
2Ask the garage to measure the resistance across the CAN pins at the diagnostic socket with the ignition off. That one reading immediately narrows the problem to a short, a break or a lost terminator.
3Declare any aftermarket electrics. Trackers, alarms, dashcams and towbar wiring spliced into the bus are a leading cause and the garage will find it much faster if they know to look.
4Expect this to be diagnosed by elimination rather than by a code reader. Disconnecting modules one at a time is slow, and the labour is often the bulk of the bill.
5Do not accept a string of replaced modules. If several modules have codes, one of them is usually the cause and the rest are victims.

Can you keep driving?

With the main network down, systems across the car cannot coordinate and you cannot rely on any of them. Depending on what has failed you may lose stability control, anti-lock braking, power steering assistance, the instruments and the lights, and the engine may cut out. A car that could shed several safety systems at once, without warning, should be recovered rather than driven.

Will it fail an MOT?

A network failure is not one MOT defect, it is a collection of them, and this is where the cost of a bad electrical fault becomes clear. Every lamp it lights is separately a major defect: the brake system warning device at 85,196 recorded failures, the airbag lamp at 84,147, stability control at 35,322, tyre pressure monitoring at 97,515 on cars first used from 1 January 2012, and the engine management lamp at 371,859. A car can genuinely collect four or five major defects from one corroded connector. Beyond that, if the tester cannot get the warning lamps to perform their bulb check at all, they cannot verify any of those systems, which is its own problem.

Common questions

How do you test a CAN bus?: The quickest meaningful test needs only a multimeter. With the ignition off and the car left to go to sleep, measure the resistance between the CAN High and CAN Low pins at the diagnostic socket — pins 6 and 14 on a standard connector. A healthy high-speed network reads around sixty ohms, because two one-hundred-and-twenty-ohm terminating resistors sit at opposite ends of the bus and appear in parallel. Roughly one hundred and twenty ohms means one terminator or one branch has dropped off. Close to zero means the two wires are shorted together. A very high reading means the bus is broken. From there an oscilloscope shows the waveform and identifies which module is misbehaving.
Can one faulty module take out the whole car?: Yes, and it is the single most important thing to understand about network faults. Every module shares the same pair of wires, so a module that fails with an internal short pulls those wires together and silences every other module on the bus. The dashboard lights up like a Christmas tree and the car may not start, which looks like a catastrophic and expensive failure. In reality one part is broken and the rest are simply unable to speak. That is why disconnecting modules one at a time until the network recovers is the standard approach.
Can a dashcam or tracker cause a CAN bus fault?: Yes, and badly installed accessories are one of the more common causes in practice. Anything spliced into the bus wiring — trackers, alarms, parking sensors, towbar electrics, some dashcam installations — introduces a new connection into a circuit designed to very specific electrical characteristics. A poorly made splice, an added length of untwisted wire, or a device that transmits when it should not can all upset the network. If the trouble started soon after something was fitted, say so, because it saves a great deal of diagnostic time.
Will U0001 fail an MOT?: Almost certainly, and probably several times over on one certificate. A network failure typically lights the ABS, airbag, stability control, tyre pressure and engine management lamps, and every one of those is a separate major defect: 85,196 recorded failures for the brake system warning device, 84,147 for the airbag lamp, 35,322 for stability control, 97,515 for tyre pressure monitoring on cars first used from 1 January 2012, and 371,859 for the engine management lamp. It is one of the few faults that can produce a failure certificate with a whole list of major defects from a single underlying cause.

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