CAN Bus on an Oscilloscope: Differential Signalling, Shorts and Termination

What this guide covers

Scoping the CAN bus — the differential CAN-H/CAN-L signalling, how shorts and termination faults appear on a scope, and why resistance testing and waveform analysis complement each other. Pairs with the CAN Bus Diagnostics electrical guide and U-codes.

CAN-H / CAN-L complementary signalling

CAN is a differential pair. On a two-channel scope (CAN-H ch1, CAN-L ch2):

• Recessive (logic 1): both lines sit together at ~2.5V; the differential is ~0V
• Dominant (logic 0): CAN-H rises and CAN-L falls; the differential grows to ~2V

The two channels move complementarily — apart for dominant, together for recessive. This complementary motion is the signature of a healthy CAN bus.

Differential concept

The receiver reads the difference between CAN-H and CAN-L, ignoring any voltage that appears equally on both (common-mode rejection). This is why CAN tolerates electrical noise — noise that couples onto both lines is cancelled out. Faults that break the differential (short between the lines, one line shorted) corrupt communication; common-mode noise does not.

Ringing / noise

Ringing (oscillation after transitions) and noise on the lines come from:

• Missing or wrong termination (reflections)
• Stub length too long on a branch
• External noise coupling

Excessive ringing corrupts edge detection and causes intermittent communication errors. A scope shows it; resistance testing does not.

Short-to-ground appearance

• CAN-L short-to-ground: CAN-L pulled down, no longer rises to the recessive level; the differential distorts or collapses. Communication fails or corrupts depending on severity.
• CAN-H short-to-ground: CAN-H cannot rise; the differential collapses.

A scope shows which line is affected; resistance testing only shows "short".

Short-to-voltage appearance

• CAN-H short-to-12V: CAN-H held high; no recessive return.
• CAN-L short-to-12V: CAN-L held high; differential inverted/collapsed.

The scope pattern identifies the fault direction, narrowing the wiring search.

Missing communication

A scope on a bus with no traffic (both lines flat at recessive, no dominant pulses) means no module is transmitting. This can be:

• All modules asleep (normal when key off)
• A module that should be awake is not (power/ground)
• The bus is disabled

A scope confirms whether traffic exists before condemning the bus.

Branch faults

On a bus with a branch to a module, a fault on the branch (short, open) can corrupt the whole bus or just drop that module. A scope at the diagnostic connector shows bus health; scope at the module branch isolates. See the CAN Bus Diagnostics guide for the network-wide vs branch-specific decision.

Termination issues

Missing or wrong-value terminating resistors cause reflections (ringing) and signal distortion. A scope shows ringing after transitions; resistance testing shows the wrong terminator value. Together they confirm both the value and the signal quality.

Why resistance testing and waveform analysis complement each other

• Resistance (key off): finds opens, CAN-H/CAN-L shorts, and missing/wrong terminators — static faults
• Scope (key on, communicating): shows dominant/recessive levels, ringing, intermittent corruption, short direction — dynamic faults

A bus that has correct resistance can still have ringing, noise, or intermittent corruption. A bus with a clean waveform can still have a marginal resistance fault under temperature. Use both. Resistance first (cheap, fast), then scope for anything intermittent or marginal.

Specification warning

CAN-H/CAN-L recessive/dominant voltage levels and termination values vary by CAN variant (high-speed CAN, fault-tolerant CAN, CAN-FD). Vehicle-specific verified specification required. Compare to a known-good waveform for that bus; do not apply one set of levels universally.

Common mistakes

• Using resistance only and missing intermittent corruption
• Not using two channels (single-channel misses the differential)
• Condemning the bus before checking module power/ground
• Ignoring ringing from missing termination
• Applying high-speed-CAN levels to a different CAN variant