CAN Bus and Network Testing

How do I check CAN bus resistance at the diagnostic connector?

With the battery disconnected, measure between pins 6 and 14 of the OBD-II connector; about 60 ohms means both terminating resistors and the wiring are intact on most high-speed CAN systems.

What should a healthy CAN bus waveform look like on a scope?

CAN high and CAN low should mirror each other around about 2.5 volts, with CAN high rising toward about 3.5 volts and CAN low falling toward about 1.5 volts when active, with clean, square edges.

What voltages should I see on CAN high and CAN low?

Short answer: On a typical high-speed CAN bus, both wires rest near the middle of the supply range when idle, and during messages CAN high rises while CAN low falls in a mirror image; check service information for the network you are testing.

High-speed CAN uses two wires that carry opposite signals. When the bus is idle, both wires sit close to the same voltage, roughly the middle of the transceiver's supply. When a module transmits a dominant bit, CAN high moves up and CAN low moves down by about the same amount. Receivers read the difference between them, which makes the bus resistant to noise.

A meter shows only an average, so it is useful to spot a wire stuck at battery voltage or ground but not the signal quality. A scope with two channels shows the mirror-image pattern clearly and reveals noise, missing signals or a wire shorted to the other.

Networks vary, and low-speed and single-wire CAN behave differently, so check service information for the vehicle. Use leads rated for the measurement and back probe carefully to avoid damaging connector seals.

What is the difference between high-speed CAN, low-speed CAN and LIN?

Short answer: High-speed CAN carries fast, critical data such as engine and brake messages, low-speed or fault-tolerant CAN often handles body functions, and LIN is a slower single-wire network for simple devices such as switches and motors.

Vehicles use several networks matched to the job. High-speed CAN connects modules that need quick data exchange, like engine, transmission, ABS and steering. It uses a twisted pair and terminating resistors.

Low-speed or fault-tolerant CAN, used on some vehicles for body and comfort systems, runs slower and can keep working with some wiring faults. LIN networks are single-wire, low-speed links connecting a master module to simple devices like window switches, mirrors, wipers or battery sensors.

Each network has its own test methods and normal waveforms. Use the wiring diagram to identify the network, then test with a scope. Disconnect the battery before unplugging modules as service information directs, and follow precautions near airbag components.

How do I test a LIN bus?

Short answer: Check power and ground at the master and slave devices, then scope the LIN wire to see whether it switches between near battery voltage and near ground in a regular pattern of messages.

A LIN bus uses one wire. When idle, it sits near battery voltage, and modules pull it toward ground to send data. On a scope you should see a clean pattern of messages switching between the high and low levels.

If the wire stays at battery voltage with no switching, the master may not be transmitting. If it stays near ground, a device or the wire may be shorted to ground. Disconnect slave devices one at a time to find a device pulling the line down.

Check power and ground for each device under load, since LIN faults are often caused by a device that lost power. Use the wiring diagram to identify the master. Back probe gently and keep leads away from moving parts and hot surfaces.