Every modern vehicle on UK roads runs on a Controller Area Network — the digital backbone that connects the engine ECU, transmission, ABS, airbag, body control and a dozen other modules.
What is the CAN bus and why does it matter?
Controller Area Network is the twisted-pair digital backbone that lets the engine ECU, gearbox, ABS, airbag, body control and instrument modules talk to each other over two wires instead of a loom the size of an arm. Every car sold in the UK from 2008 onwards uses CAN for OBD-II diagnostics on pin 6 (CAN High) and pin 14 (CAN Low).
Because every module shares the same pair of wires, one failed module or one chafed wire can take the whole network down — which is why a car with a dead CAN bus often shows a dashboard full of unrelated warning lights and refuses to talk to a scan tool at all.
CAN bus voltages — the first thing to measure
With the ignition on and the bus idle, CAN High and CAN Low both sit at roughly 2.5 V against battery ground. When a module transmits, CAN High rises to about 3.5 V and CAN Low drops to about 1.5 V — a differential of around 2 V. Both lines stuck at 0 V means a short to ground; both at battery voltage means a short to power; one line flat while the other swings means an open circuit on that wire.
Back-probe at the OBD-II socket with the meter on DC volts, then repeat at the suspect module connector. The point where the healthy 2.5 V idle disappears is the section of loom to inspect.
The 60 ohm termination test (do this first)
Switch the ignition OFF, set the meter to resistance and measure between pin 6 and pin 14 at the OBD-II socket. A healthy high-speed CAN network reads 60 ohms — the two 120 ohm terminating resistors, usually inside the engine ECU and one other module, sitting in parallel.
About 120 ohms means one terminator or one branch has gone open circuit. Near 0 ohms means CAN High and CAN Low are shorted together, commonly where a loom rubs through on a bracket. An open or infinite reading means the wiring is broken or the socket has lost pins. This one test rules in or out most no-communication faults in under a minute.
Step-by-step CAN bus troubleshooting
Confirm power and ground at the diagnostic socket first: battery voltage on pin 16 to pin 4 and to pin 5. Then run the 60 ohm termination test. If resistance is correct, check the idle voltages, then start unplugging modules one at a time — a module dragging the bus down will restore normal voltages the moment it is disconnected.
Inspect the usual physical trouble spots: door and tailgate loom concertinas, boot lid grommets, water ingress into rear light clusters and boot-floor modules, and any aftermarket alarm, tow bar or stereo wiring spliced into the bus. Corroded ground points cause more CAN faults than failed modules do.
OBD-II diagnostic requests over CAN
OBD-II runs on top of CAN as ISO 15765-4, with the tester sending requests to identifier 0x7DF and modules replying on 0x7E8 upwards. Mode 01 returns live data PIDs such as engine RPM, coolant temperature and fuel trims; mode 03 returns stored fault codes; mode 07 pending codes and mode 09 vehicle information including the VIN.
A generic dongle reads emissions-related data on any compliant car, but body, chassis and comfort modules usually need marque-specific software — which is why a cheap scanner can be fault-free on an engine light and blind to the ABS module beside it.
Frequently asked questions
What resistance should a CAN bus read?
60 ohms between OBD-II pin 6 and pin 14 with the ignition off. That is the two 120 ohm terminating resistors in parallel. 120 ohms means one terminator or branch is open, near 0 ohms means the two lines are shorted together.
What voltage should CAN High and CAN Low be?
Both idle at around 2.5 V with the ignition on. When a module transmits, CAN High rises to about 3.5 V and CAN Low falls to about 1.5 V.
What causes a CAN bus failure?
Chafed or water-damaged wiring in door and tailgate looms, corroded earth points, water ingress into boot-mounted modules, a single failed module pulling the bus down, and badly spliced aftermarket alarms, tow bars or stereos.
Can I diagnose CAN bus faults with a cheap scanner?
A cheap scanner tells you whether communication is possible at all, but it will not show body or chassis modules. For real CAN fault-finding you need a multimeter for voltages and termination, and ideally an oscilloscope to see the waveform.
Which OBD2 pins carry CAN?
Pin 6 is CAN High and pin 14 is CAN Low on the 16-pin OBD-II connector, standardised under ISO 15765-4.