CAN Bus Physical Layer and Termination

Before you can diagnose a CAN bus fault, you need to understand what the signals look like at the physical level. CAN bus is not just data on a wire — it is a precisely engineered electrical system with specific voltage levels, impedance requirements, and termination that must all be correct for communication to work. Understanding the physical layer turns CAN bus diagnosis from guesswork into measurement.

CAN bus uses two wires — CAN High (CAN-H) and CAN Low (CAN-L). Both wires idle at approximately 2.5 volts when no module is transmitting. This is the recessive state. When a module transmits a dominant bit, CAN-H rises to approximately 3.5 volts and CAN-L drops to approximately 1.5 volts. The receiving module reads the voltage difference between the two wires — not the voltage of either wire alone. This differential design is the reason CAN bus resists electrical noise. External noise affects both wires equally, so the difference between them stays the same. If noise raises both wires by 0.5 volts, CAN-H goes to 4.0V and CAN-L goes to 2.0V — the difference is still 2.0 volts and the data is readable.

Each end of the CAN bus backbone has a 120-ohm resistor between CAN-H and CAN-L. These terminating resistors serve two purposes — they set the correct impedance for the transmission line and they prevent signal reflections at the ends of the bus that would corrupt data. Two 120-ohm resistors in parallel measure 60 ohms. This is the single most important measurement in CAN bus diagnosis. At the DLC with the battery disconnected, pins 6 (CAN-H) and 14 (CAN-L) should read 60 ohms. If you read 120 ohms, one terminator is open. If you read 40 ohms, there is an extra termination somewhere. If you read near zero, the bus is shorted.

High-speed CAN bus on most vehicles is wired as a backbone with short stubs branching to individual modules. The backbone runs the length of the vehicle with the two terminating resistors at each end. Module connectors tap into the backbone through short branch wires. This topology means a fault anywhere on the backbone affects every module on the bus. A fault on a short stub to one module may only affect that module — or it may drag down the entire bus if the module shorts the bus lines internally.

The standardized OBD-II diagnostic link connector has 16 pins. Pin 6 is CAN-H and pin 14 is CAN-L for the high-speed CAN bus. Pin 4 is chassis ground and pin 16 is battery positive — always present. Pins 1 and 3 are sometimes used for manufacturer-specific single-wire CAN or additional bus connections. Pin 5 is signal ground. Not all pins are populated on every vehicle. The DLC is your access point for CAN bus testing because the bus backbone connects to it — you can measure the health of the entire high-speed network from this single connector under the dash.