Field Guide · For Techs

CAN Bus Diagnosis
when the network goes quiet

High-speed CAN is two twisted wires — a differential pair, terminated at both ends. When it drops, half the car lights up with lost-comm codes and the parts cannon is useless. Here's how to corner the fault with resistance, voltage, and the scope — and the one trap that makes a stone-dead bus read a perfect 60 ohms.

Start here

What a healthy bus is made of

  • Two 120 Ω terminators, one at each end. In parallel they read 60 Ω across CAN-H and CAN-L. That single number, key off, is your fastest first test.
  • It's differential. At rest (recessive) both wires sit at ~2.5 V. To send a dominant bit a node pulls CAN-H up to ~3.5 V and CAN-L down to ~1.5 V — a ~2 V swing that mirrors around 2.5 V. Noise hits both wires equally and cancels; that's why it's twisted.
  • Data lives in the gap, not the level. The receiver reads CAN-H minus CAN-L. Kill the gap — short the wires together, or pin one to ground — and there's no data even if a wire still "has voltage."
Rule #1: resistance first (key off), then voltage (key on), then the scope. Each catches faults the previous one is blind to — and one fault sails right past the ohmmeter. The simulator below shows you exactly how.
InteractiveBus fault simulator — inject a fault, read the meters
Pick a fault · watch the ohmmeter, the wiring and the scope react

Eight faults you'll actually chase

Tap a fault. The DMM shows the key-off resistance across CAN-H / CAN-L, the diagram highlights where it lives, and the scope shows what the differential does. Watch what happens to resistance on the shorts-to-ground — that's the trap.

DMM · Ω · key off
60 Ω
across pins 6 & 14
Two-wire busterminated · 60 Ω
CAN bus wiring diagram with fault highlighted H L 120Ω 120Ω ECM TCM BCM +B
Scope · differential2.5 V rest · ±1 V swing
Oscilloscope view of CAN-H and CAN-L 3.5 2.5 1.5
CAN-H CAN-L

Bus terminated correctly healthy

Three testsIn order, because each is blind to something
  • 1 · ResistanceKey off, across CAN-H / CAN-L — 60 Ω good · 120 Ω = a terminator gone · very high = both gone · ~0 Ω = wires shorted together
  • 2 · VoltageKey on, each wire to ground — both ~2.5 V at rest; a wire pinned near 0 V or up at B+ is a short the ohmmeter missed
  • 3 · ScopeBoth wires, differential — clean mirrored swing = alive; collapsed or flat = dead; ringing = reflections from lost termination
Read the metersWhat each reading is really saying

60 · 120 · very high · 0

Resistance
  • 60 Ω = both terminators alive. 120 Ω = one terminator gone — but that's a diagnosis, not a broken wire: terminators often live inside two specific end modules.
  • Very high (tens of kΩ) = both terminators lost — you're reading node input impedances, not a clean open. ~0 Ω = CAN-H and CAN-L shorted together.

The 2.5 V idle

Voltage
  • Key on, bus at rest: both wires ~2.5 V to ground. Active traffic pulls the average of CAN-H up a little and CAN-L down a little.
  • A wire sitting at ~0 V = shorted to ground. At ~5 V or battery = shorted to power (and it may have cooked transceivers). This is the test that catches what resistance can't.

The mirrored swing

Scope
  • Healthy: CAN-H and CAN-L are mirror images around 2.5 V. Lay them on top of each other and they should be symmetrical.
  • Ringing / overshoot after each bit = lost termination (reflections). Collapsed to one flat line = shorted. One wire flat, one moving = a wire pinned to ground or power.

Architecture & the DLC

Caveat
  • 60 Ω at the DLC only proves the DLC's bus. Modern cars gateway several buses; sub-networks (chassis, body, infotainment) may not appear at pins 6 & 14 at all.
  • Pins 6/14 are the OBD-II high-speed CAN convention, not universal. Check the diagram for where your bus lands and where its terminators live before you condemn wiring.
Where CAN diagnosis goes wrong

The traps

  • Trusting 60 Ω on a dead bus. A short to ground or power leaves termination intact, so resistance still reads ~60 Ω while the bus is stone dead. Resistance can't see it — scope or voltage-test to catch it.
  • Measuring resistance key-on. The number only means something with the bus asleep and modules powered down; live, you're reading driven voltages, not termination.
  • Calling 120 Ω a broken wire. It's a missing terminator — most often a terminating module that lost power or ground. Chase the end, not the middle.
  • One babbling node kills everyone. A single module stuck dominant, or spewing errors, drives the whole bus off — every other module logs lost-comm. Disconnect nodes one at a time; when the bus wakes up, you found it.
  • Piercing the twisted pair. Back-probe or use the connector — a pierced, un-sealed CAN wire corrodes and becomes next winter's intermittent.
InteractiveSelf-check — tap to reveal

Q1. Key-off resistance across CAN-H / CAN-L reads 60 Ω, but you have a fistful of U-codes and a module that won't talk. Wiring or module?

Module. 60 Ω means both terminators are intact and the pair isn't shorted or open — the physical layer is good. Lost-comm codes with good termination point at a node that lost power or ground, or one that's stuck/babbling. Check power and ground at the silent module; if the whole bus is down, start disconnecting nodes to find the one dragging it off.

Q2. Resistance reads a healthy 60 Ω, yet nothing on the bus communicates. What's your next move and why?

Scope it / voltage-test it — don't trust the 60 Ω. A CAN wire shorted to ground or to power leaves the terminators intact, so resistance still reads ~60 Ω on a completely dead bus. Key on and measure each wire to ground: healthy is ~2.5 V. A wire pinned near 0 V (short to ground) or up at battery (short to power) is the fault the ohmmeter can't see.

Q3. You read 120 Ω key-off. Is that a broken wire?

Not necessarily — it's one terminator missing. Two 120 Ω terminators in parallel make 60 Ω; lose one and you read 120 Ω. Since terminators frequently live inside end modules, the usual cause is a terminating module that lost power/ground, or a break isolating one end's resistor — not a random wire fault in the middle. Identify which end lost its 120 Ω and go there.

MethodCorner a dead bus without guessing
  1. Resistance, key offAcross CAN-H / CAN-L. 60 Ω good, 120 Ω one terminator, very high both, ~0 Ω shorted. Ten-second first cut.
  2. Voltage, key onEach wire to ground, ~2.5 V at rest. Pinned to 0 V or to battery is the short resistance couldn't see.
  3. Scope the differentialBoth wires together. Mirror-image swing = alive. Collapsed, flat, or ringing tells you shorted, dead, or unterminated.
  4. Divide the harnessUnplug modules one at a time. When resistance normalizes or the bus wakes up, the last thing you touched owns the fault.
  5. Confirm the module, not just the wireGood physical layer plus lost-comm = power/ground/software at the node. Prove the module before you sell a harness.
The unfair advantage

Bus down and the readings don't add up?

60 Ω but no comms, a U-code storm across three modules, one wire reading 4 volts — tell Kate the resistance, the key-on voltages on each wire, which modules dropped, and the architecture, and she'll tell you whether it's the pair, a terminator, or a node — and exactly which one to unplug next. She knows which bus lands on which DLC pins and where the terminators hide. No guessing, no "consult a professional."