Network Communication: CAN Bus and Module Communication Basics

Why Modules Need a Network

Modern vehicles contain dozens of electronic control modules — the Engine Control Module (ECM), Transmission Control Module (TCM), Body Control Module (BCM), Anti-lock Brake System (ABS) module, and many others. Each module needs information from sensors and from other modules. Rather than running a separate wire between every pair of modules (which would add hundreds of wires and significant weight), engineers designed a shared communication network called a Controller Area Network (CAN bus). With CAN, all modules share the same pair of wires, passing data back and forth in structured digital messages.

How CAN Bus Works

A CAN network uses two wires: CAN High (CAN-H) and CAN Low (CAN-L). These two wires always work together as a differential pair, meaning the voltage on one wire rises while the voltage on the other falls. When no module is transmitting (the bus is idle), both wires rest at approximately 2.5 V relative to ground. When a module sends a dominant bit (logic 0), CAN-H rises toward about 3.5 V and CAN-L falls toward about 1.5 V. When a recessive bit (logic 1) is sent, both wires return toward 2.5 V. This differential design makes CAN resistant to electrical noise.

Modules read every message on the bus but act only on the ones addressed to them. Each message carries an identifier that tells other modules what data is included — for example, engine RPM, vehicle speed, or brake pressure.

High-Speed vs. Low-Speed CAN

Most vehicles use more than one CAN network running at different speeds. High-speed CAN (typically 500 kbps) handles safety-critical systems: ECM, TCM, ABS, and airbag modules that need rapid data exchange. Low-speed CAN (typically 125 kbps) handles comfort and convenience systems: BCM, instrument cluster, door modules, and HVAC. A gateway module (sometimes the BCM or a dedicated module) bridges these separate networks, translating messages between them. On wiring diagrams you will see each network labeled with its speed and bus name.

Terminating Resistors

To prevent signal reflections that would corrupt data, a terminating resistor of 120 Ω is placed at each end of the CAN bus. Because these two resistors are wired in parallel across the same two nodes, a technician measuring resistance across CAN-H and CAN-L with the ignition off and both terminators intact will read approximately 60 Ω (120 Ω ∥ 120 Ω = 60 Ω). A reading near 120 Ω means one terminator is open; a reading near 0 Ω means CAN-H and CAN-L are shorted together. Knowing these values is your first diagnostic checkpoint.

U-Code DTCs and Network Faults

When a module cannot communicate on the network, the vehicle's diagnostic system stores a U-code (network communication DTC). U-codes follow SAE J2012 naming convention just like P, B, and C codes. A U0001 indicates a high-speed CAN bus communication fault; U0100 through U019F codes typically point to a specific module that is not responding. Always retrieve U-codes from every module using the scan tool's module communication or network test function, not just the ECM. A module that has lost power may store no codes at all — that silence is itself a clue.

Using a Scan Tool for Network Diagnostics

1. Connect the scan tool and select the correct vehicle year, make, and model.

2. Run the module communications test (sometimes called a "network health" or "all-systems scan"). The scan tool polls every expected module.

3. Note any module shown as "No Response" or "Not Detected."

4. Retrieve U-codes from all responding modules and record them.

5. Cross-reference each U-code with the OEM wiring diagram to identify which bus segment or which module's power, ground, or bus wiring is suspect.

DMM Measurements on the CAN Bus

With the ignition on and the engine off:

• Bias voltage: Backprobe CAN-H at the OBD-II connector (pin 6 for high-speed CAN-H, pin 14 for CAN-L). Set the DMM to DC volts. Expected readings: CAN-H nominally 2.5–3.5 V, CAN-L nominally 1.5–2.5 V at idle communication states.
• Terminating resistance: Turn the ignition OFF and disconnect the vehicle battery before measuring resistance to protect the DMM and avoid false readings from module internal circuits. Measure across CAN-H and CAN-L. Nominal result: ~60 Ω.

Safety Points

• Eye protection (safety glasses) must be worn in the shop at all times.
• Before disconnecting any module or connector, follow the manufacturer's procedure for disabling the airbag system if working near supplemental restraint components. Accidental deployment can cause serious injury.
• Hybrid and electric vehicle high-voltage systems are completely separate from the CAN bus low-voltage network, but always confirm which vehicle type you are working on and follow HV lockout procedures if applicable.
• When using a DMM on a live circuit, confirm the meter leads and range are set correctly before probing to avoid a short across CAN-H and CAN-L, which can damage modules.
• Never pierce wire insulation to backprobe; use proper backprobe pins to avoid damaging the wire and creating a new fault.