Failed bus terminating resistor

One of the two resistors at the ends of a data bus is missing, out of value, or no longer connected.

A CAN bus is a transmission line rather than a pair of wires, and a transmission line has to be terminated. A 120 ohm resistor sits at each physical end of the backbone, absorbing the signal when it arrives so that it does not reflect and travel back — and a reflection arriving late on top of the next message is exactly what corrupts traffic.

That gives the measurement the whole family turns on. Two 120 ohm resistors in parallel are 60, so a healthy high speed bus reads about 60 ohms across CAN-H and CAN-L with the ignition off and the network asleep. Which makes the readings unusually legible:

• About 60 — both terminators present and the backbone intact.
• About 120 — one terminator is missing, or the backbone is broken between the two of them.
• Open circuit — the pair is severed, or both terminators are gone.
• Near zero — the two conductors are shorted together.

One consequence surprises people and it is the point of this entry: the terminators are usually not separate components. They are inside two modules at the ends of the backbone — commonly the engine module and one other. So "replace the terminating resistor" frequently means replacing a module, and a steady 120 ohm reading is either a broken backbone or a module that has stopped terminating.

The figures above are high speed CAN figures and do not transfer. Medium and low speed networks are terminated differently — often at every node, at quite different values — so applying the familiar number to them condemns healthy wiring. Use the manufacturer's figure for the network in question. The one reading that holds everywhere is that near zero is wrong on any bus.

A missing terminator does not always stop the network. It frequently degrades it instead, producing errors under traffic while every static test passes — which is why this cause turns up behind performance codes as often as behind outright failures.