CAN bus physical layer checks

Before you blame a module, prove whether the network wiring looks healthy. These checks are intentionally simple and safe — they won’t replace OEM procedures, but they catch the common real-world failures fast.

What this guide is for

Multiple U‑codes across modules
Scan tool can’t connect / intermittent comms
“Christmas tree dash” after low battery, water ingress, or recent electrical work

Safety note: Don’t pierce insulation. Back-probe at connectors when possible. If you’re unsure, stop and get a wiring diagram.

Test 1: CAN resistance (key off)

Key off. Let the car go to sleep (wait a few minutes).
Measure resistance across CAN‑H and CAN‑L at a convenient network connector (often OBD).

What you hope to see

~60 Ω (two 120 Ω terminators in parallel). Some vehicles may differ, but 60 Ω is a strong “healthy baseline”.

What bad results suggest

~120 Ω: one terminator missing / open circuit / a section disconnected
Very low Ω: short between lines or to power/ground
Open circuit: broken wiring / connector / missing network connection

Test 2: Voltage bias (key on)

With key on (engine off), measure each line to ground (not line-to-line). Many systems sit around ~2–3 V when idle.

CAN‑H to ground: typically higher than CAN‑L
CAN‑L to ground: typically lower than CAN‑H

If a line is stuck near 0 V or near battery voltage, suspect a short to ground/power or a damaged module pulling the bus.

Fast isolation strategy (real-world)

Start with known trouble spots: water ingress areas, fuse boxes, boot/trunk looms, door looms.
Unplug one suspect module at a time (especially aftermarket modules, towbar modules, audio amps).
Recheck resistance/voltage after each unplug. A big jump back towards “normal” points to the culprit branch/module.

Common causes that mimic “dead ECU”

Low battery / poor grounds (fix voltage first)
Corrosion in a connector causing intermittent bus open
Aftermarket devices spliced into CAN
Water ingress into a module that then drags the network down