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CAN bus networks use two termination resistors (typically 120Ω each) placed at the physical ends of the bus to prevent signal reflections and ensure clean, reliable communication. Without proper termination, waveforms distort (ringing, overshoot), bits corrupt, and modules drop offline—triggering U-codes and lost communication faults. Measuring total network resistance between CAN High and CAN Low is one of the fastest, non-invasive ways to detect major wiring faults (shorts, opens, missing termination) without guessing or unplugging modules randomly.

Key concept: The two 120Ω resistors are in parallel across the bus, so measured resistance at any point should be approximately **60Ω** (120Ω || 120Ω = 60Ω) on a healthy high-speed CAN network. Abnormal readings almost always indicate a physical bus integrity problem.

How to Measure CAN Termination Resistance Correctly

1. Ignition OFF & vehicle asleep — Key out, wait 30–60 minutes for modules to fully sleep (awake transceivers can lower resistance or skew readings on some networks).
2. Disconnect battery negative (recommended) — Prevents any residual power or module wake-up from affecting measurement.
3. Access the bus — Measure between CAN High (OBD-II pin 6) and CAN Low (pin 14) at the DLC (diagnostic connector) — most convenient point. Alternatively, use a known splice or junction if DLC inaccessible.
4. Set DMM to ohms — Low range (200Ω or auto). Ensure probes have good contact; wiggle slightly to check for intermittents.
5. Read & record — Normal high-speed CAN: 54–66Ω typical (close to 60Ω). Variations depend on exact resistor values and network length.

How to Interpret CAN Resistance Readings

• Normal: ~54–66Ω (close to 60Ω) — Two 120Ω terminators in parallel; bus integrity good. Proceed to voltage/signal checks if U-codes persist.
• Too high: ~120Ω or higher — One terminator missing, open circuit in one branch, or broken wire between measurement point and a terminator. Communication may work intermittently or fail completely.
• Too low: <40Ω or near 0Ω — Short between CAN H and CAN L, extra/misplaced termination, or a module transceiver dragging the bus low (stuck dominant). Bus will be flooded with errors.
• Infinite/open (OL) — Complete open circuit, disconnected network, both terminators missing, or measurement point isolated from bus. No communication possible.

Where Termination Resistors Live

Termination is rarely a separate plug-in part—it’s built into the network design:

• Most common: Inside two key modules at opposite ends of the bus (e.g., PCM + ABS module, or gateway + instrument cluster).
• Some vehicles: Dedicated termination connectors or resistors in splice packs/junction blocks.
• High-speed CAN (powertrain): Usually 120Ω at each end.
• Medium/low-speed CAN: May use different values or single termination (check service info).
• Use wiring diagrams to locate exact termination points—critical for isolation.

Next Steps After an Abnormal Reading

Do not unplug random modules immediately—disconnection changes resistance and can mask the fault. Instead:

• Use wiring diagram to identify network splits, gateways, and branch points.
• Isolate at known junctions or module connectors — unplug one branch at a time, remeasure resistance until it returns to normal (~60Ω with one terminator if other end still connected).
• When resistance corrects after unplugging → fault is in that disconnected branch (short, open, bad terminator/module).
• Inspect harness routing, connectors, splices in suspect area for damage, corrosion, chafing, or water intrusion.
• After repair, remeasure resistance, then confirm communication with CAN signal testing and full scan.

Termination resistance is a quick gatekeeper test—if abnormal, you’ve found a major bus integrity issue before wasting time on modules or signals. Normal resistance? Move to power/ground and waveform checks (U-code diagnosis workflow).

Updated March 2026 – Part of our Complete Guide to CAN Bus & Network Diagnostics.