CAN Bus: Diagnosing CAN-H/CAN-L, Termination and Communication Faults

What this guide covers

CAN bus diagnostics — the Controller Area Network that links vehicle modules. CAN-H, CAN-L, termination, dominant/recessive signalling, shorts, opens, branch faults, and how to separate network-wide from branch-specific communication loss. Pairs with the CAN Bus Waveform oscilloscope guide and U-codes.

CAN-H and CAN-L

CAN is a differential pair: CAN-H (High) and CAN-L (Low). Data is carried as the difference between the two lines, which makes CAN robust against common-mode noise. Both lines carry the same data; the receiver reads the differential, ignoring voltage that appears equally on both.

Termination concept

CAN buses use terminating resistors (commonly 120 ohms each) at the physical ends of the bus to absorb signals and prevent reflections. With two resistors, the static resistance across CAN-H/CAN-L reads ~60 ohms. But topology varies — some buses use one resistor, some use different values, and modern vehicles have multiple CAN sub-networks (drive-train CAN, comfort CAN, FlexRay on some). Do not assume 60 ohms universally; interpret in the vehicle's context.

Dominant / recessive signalling

• Recessive (logic 1): CAN-H and CAN-L sit at the same potential (~2.5V); the differential is near zero
• Dominant (logic 0): CAN-H rises and CAN-L falls; the differential is significant (~2V)

On a scope (see the CAN Bus Waveform guide), the two lines move apart for a dominant bit and come together for a recessive bit. This complementary motion is the signature of healthy CAN.

Shorts

• CAN-H to CAN-L short: the two lines merge; differential collapses; no communication
• CAN-H or CAN-L to ground/12V: pulls one line; differential distorts; communication corrupts or fails
• Short to another circuit: intermittent corruption, often load-dependent

Open circuits

• Open on the backbone: modules downstream of the open lose communication; modules upstream still talk
• Open on a module branch: only that module drops off
• Open on one line (CAN-H or CAN-L): the differential fails; communication on that branch stops

Module branch faults

If a single module reports "no communication" while others on the same bus are fine, the fault is local to that module: its branch wiring, its power/ground, or the module itself. Check the module's power and ground before the bus — a module with no supply drops off the bus just as cleanly as a cut wire.

Network-wide vs branch-specific communication loss

• Multiple modules down → backbone (main bus) fault, a shared power/ground, or a gateway fault
• One module down → that module's branch, power/ground, or the module itself
• Intermittent / load-dependent → short to another circuit, loose connection, or ground issue

This distinction — whole-network vs single-module — is the first branch in CAN diagnosis.

Resistance testing context

With the battery disconnected (modules off), measure across CAN-H and CAN-L:

• ~60 ohms → two 120-ohm terminators present (typical high-speed CAN — but confirm topology)
• 120 ohms → one terminator missing or open
• Very low / short → CAN-H to CAN-L short
• Open (very high) → open backbone or both terminators missing

Resistance finds opens, shorts, and missing terminators. It does not find intermittent corruption — for that, use a scope.

Oscilloscope testing

A scope on CAN-H and CAN-L (two channels) shows:

• The complementary dominant/recessive motion
• Ringing/noise that corrupts signals
• Distorted levels from a partial short or a loaded line
• Intermittent dropouts that resistance testing misses

See the CAN Bus Waveform guide for the waveform approach.

Common diagnostic mistakes

• Assuming 60 ohms is universal without checking the vehicle's topology
• Condemning a module for "no communication" before checking its power and ground
• Not distinguishing network-wide from single-module loss
• Using resistance only and missing intermittent corruption
• Overlooking a short to another circuit (load-dependent)
• Ignoring the gateway when multiple sub-networks are involved