Looking for the complete picture? Explore our CAN Bus and Network Diagnostics: The Complete Guide to U-Codes for an in-depth guide.
Testing CAN High and CAN Low signals is essential for diagnosing communication faults (U-codes, lost modules, intermittent network dropouts). There are two practical levels: a quick multimeter sanity check to spot obvious shorts/opens or bias issues, and a definitive oscilloscope check to verify clean, active communication. Always start with power/ground stability and termination resistance (termination explained) before signal testing—many “bus” problems are upstream voltage or termination faults.
Key principle: CAN is differential—data lives in the voltage difference between CAN H and CAN Low. Measure both wires; a fault on one wire destroys communication, even if the other looks “normal.”
Tools Needed
- Digital multimeter (DMM) — DC volts for bias check
- Oscilloscope (automotive-grade preferred) — For waveform, noise, and timing analysis
- Backprobe pins or breakout leads (backprobing safely)
- Service info: DLC pinout (CAN H = pin 6, CAN L = pin 14 on OBD-II), network type (HS-CAN, MS-CAN, etc.), expected bias voltages
- Optional: Scan tool — Wake modules, trigger communication, monitor PIDs
Multimeter Checks (Quick Screening – Ignition ON)
- Prepare vehicle — Ignition ON (engine off or running); wait for modules to wake (some networks need key cycle or accessory on).
- Backprobe at DLC or access point — Measure CAN High (pin 6) to chassis ground, then CAN Low (pin 14) to chassis ground.
- Expect reasonable bias voltages — Typical high-speed CAN (key on, bus active): – CAN High: ~2.5–3.5V (often ~3.0V) – CAN Low: ~1.5–2.5V (often ~2.0V) – Differential (H – L): ~0.5–2V Exact values vary by vehicle—check service info for spec.
- Interpret readings: – Both ~2.5V with no delta = bus idle/recessive (normal if no traffic). – Pinned at 0V (either wire) = short to ground or stuck low transceiver. – Pinned at battery voltage (~12V) = short to power. – One wire normal, other pinned/flat = open or short on faulty wire. – Both 0V or both battery = major power/ground issue to network.
Oscilloscope Checks (Best Practice – Definitive Waveform Analysis)
- Setup safely — Backprobe CAN High and CAN Low at DLC or module connector. Scope ground to clean chassis/battery negative. Use short leads to minimize noise pickup.
- Configure scope — – Voltage: 1–2V per division – Time base: 1–5 ms/div for general traffic; zoom in for bit timing – Trigger: Normal/edge on rising/falling CAN H or L – Coupling: DC
- Wake the network & capture — Key on, cycle ignition, operate systems (turn signals, windows, scan tool requests) to generate traffic. Look for activity on both channels.
- Analyze waveforms — Healthy high-speed CAN: – Mirrored square waves: CAN H rises ~3.5V, CAN Low drops ~1.5V during dominant bits. – Clean edges, no excessive ringing/overshoot. – Recessive (idle): both near 2.5V. – Dominant bits: clear 0s; recessive 1s not pulled low.
Common Waveform Problems & Likely Causes
| What You See on Scope | Likely Cause | Next Move |
|---|---|---|
| One line flat, the other active | Open circuit on the flat line (broken wire, disconnected, corrosion) | Inspect connectors, splices, harness damage; check continuity |
| Both lines flat (no activity) | No traffic (modules asleep/no wake), network open, or total power loss | Wake network (key cycle, scan tool), check power to gateway, verify resistance |
| Heavy noise/distorted edges/ringing | Poor grounding, corrosion, water intrusion, EMI, or bad termination | Inspect grounds/splice packs; see intermittent faults |
| Stuck dominant (bus held low) | Short to ground, stuck transceiver, or module holding dominant | Isolate by unplugging suspected module branches; retest |
| Clipped or offset waveforms | Short to power or ground on one wire, bias voltage issue | Check multimeter bias voltages; isolate shorts |
| Excessive AC ripple on signals | Alternator diode leakage or EMI from other sources | Test alternator ripple (alternator test) |
Verification & Next Steps
After repair (wiring fix, connector clean, module power/ground): – Retest waveforms — clean, mirrored, active traffic. – Rescan full vehicle — confirm no U-codes, all modules responding. – Road test with scan tool monitoring — stable communication, no dropouts. If waveforms normal but faults persist, check termination (termination explained), power/ground, or module-specific issues.
Updated March 2026 – Part of our Complete Guide to CAN Bus & Network Diagnostics.