CAN bus faults that are actually harness faults
A CAN fault code rarely means the module is bad. It usually means one of four harness conditions: the pair is no longer twisted, the termination resistor is missing or doubled, a pin has lost tension in a connector, or the shield has lost its ground. All four are measurable.
The four harness conditions that break CAN
| Condition | Symptom | Measurement |
|---|---|---|
| Termination incorrect | Bus will not communicate at all, or only with ignition cycling | 60 ohm across CAN-H and CAN-L with the bus powered down (two 120 ohm resistors in parallel) |
| Pair untwisted or opened | Intermittent dropouts, worse in damp or hot conditions | Visually check for repairs that separated the pair; the twist must continue to within 50 mm of the connector |
| Pin tension lost | One module intermittently missing; codes that move between modules | Drag test each pin; a terminal that backs out under light pull has lost its lock |
| Shield not grounded | Noise-related faults, often after body or radio work | Continuity from shield to the specified ground point, below 1 ohm |
Test order matters
Measure termination first, with the battery disconnected and all modules asleep. A reading near 60 ohm is healthy; 120 ohm means one terminator is missing or one module is offline; near 0 ohm means the pair is shorted together, which is a harness fault almost every time.
Then check the pair physically. Repairs made with two separate wires, or with a length of unpaired wire, destroy the differential signal: the bus may work at idle and fail under electrical load. Any repair to a CAN pair should use twisted wire of the correct pitch, with the twist maintained up to the terminal.
Do not measure CAN resistance with the ignition on. Modules bias the bus and the reading becomes meaningless.
Where CAN faults come from in practice
- A tow-bar or accessory harness tapped into the body loom near the CAN pair
- Water in a connector on the body loom, bridging CAN-H to CAN-L
- A body repair that pinched the loom behind a trim panel
- An aftermarket radio adapter that re-routed the pair around the head unit
- Corrosion at a floor splice after a windscreen or heater matrix leak
What a replacement has to preserve
A rebuilt harness must reproduce the pair exactly: same twist pitch, same impedance, same routing away from high-current feeds, and the same shield termination. This is where a cheap replacement shows up first — the vehicle starts, then throws communication codes under load two weeks later.
When we build a harness with a CAN pair, the pair is drawn as a pair in the circuit list and tested for continuity at both conductors plus shield, not just for end-to-end continuity.
Questions on this fault
What resistance should I read across CAN-H and CAN-L?
About 60 ohm with the bus powered down, because the two 120 ohm terminators sit in parallel. 120 ohm means one terminator is missing; near 0 ohm means the pair is shorted.
Can a bad ground cause CAN faults?
Yes. CAN transceivers reference the bus to ground, and a ground offset of more than about 1 V between modules corrupts the differential signal. Check module grounds before condemning the bus wiring.
Do I need twisted wire to repair a CAN pair?
Yes. Untwisted wire loses common-mode noise rejection, and the fault returns under electrical load. Use twisted automotive wire of the same pitch and keep the twist to within 50 mm of the terminal.
Where this fault is common
These platforms come up most often for this repair. Each page lists the assemblies, chassis codes and known failure points.
Send the part number, get a fitment answer
Photo of the label plus the VIN is enough for most platforms. Quote in 24 hours, sample in 7–10 days.