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TRN-AUTO19-07 · Lesson 5 · 18 clock hours · theory

Network Communications Diagnostics: CAN Bus and Module Communication

Objective. Students will describe the function and architecture of a CAN bus network, use a scan tool to identify U-code communication faults, and use a DSO to measure CAN bus voltage levels and verify network integrity on an assigned vehicle or trainer.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Network Communications Diagnostics: CAN Bus and Module Communication

Modern vehicles are rolling computer networks. Instead of running a dedicated wire from every sensor to every control module, manufacturers connect dozens of modules on shared data buses. The Controller Area Network (CAN) is the dominant standard. As a technician, you must understand how the network is built, how faults appear, and how to measure the electrical signals that carry the data.

How CAN Bus Works

CAN uses two wires: CAN High (CANH) and CAN Low (CANL). Both wires carry the same signal, but they are mirror images of each other — this is called differential signaling. The diagnostic system reads the difference in voltage between the two wires, not the voltage of either wire alone. This design cancels out electrical noise from ignition systems, motors, and other interference sources — a major reason CAN became the industry standard.

In a recessive (no-data) state, both wires sit near 2.5 V, so the differential voltage is approximately 0 V. When a module transmits a dominant bit (a logical zero), CANH rises to approximately 3.5 V and CANL drops to approximately 1.5 V, creating a differential of approximately 2 V. These voltage levels are the specification you will compare your DSO measurement against.

Network Architecture

A vehicle typically has more than one CAN segment running at different speeds. A high-speed CAN bus (often 500 kbps) connects powertrain and safety modules — engine control module (ECM), transmission control module (TCM), anti-lock brake module (ABS), and body control module (BCM). A medium- or lower-speed bus may serve body and convenience modules. A gateway module (sometimes called a central gateway or body control module, depending on the vehicle) bridges segments, translating messages between buses so that, for example, the instrument cluster can display engine speed data from the high-speed powertrain bus.

Each module on a network has a unique node address and transmits specific messages on a defined schedule. If a module stops transmitting, other modules that depend on its data may store communication fault codes.

U-Codes and Cascade Faults

SAE J2012 defines U-codes as network communication diagnostic trouble codes. A U0xxx code typically means a module is not communicating — for example, U0100 indicates lost communication with the ECM. When one module fails or loses power or ground, every other module that expects data from it will set its own U-code. This cascade effect means you may see eight or ten U-codes after one module fails. Your job is to find the root cause — the module that is actually offline — rather than replacing every module that logged a fault.

A full network scan with a professional scan tool will show you which modules respond and which do not. Start with the module that is not responding, not the module that set the most codes.

Using the Scan Tool for Network Diagnostics

Perform a complete network scan, not just a powertrain scan. Record every U-code and note which modules are listed as not communicating. Pull up the vehicle's network topology diagram in the service information and locate each affected module. Check whether the non-communicating modules share a common power supply, ground, or network segment — a pattern often points to a single failed component such as a blown fuse, corroded ground, or failed gateway.

DSO Measurement of CAN Bus Signals

An oscilloscope (DSO) lets you see the actual waveform rather than just a code. Use an OBD-II breakout adapter to safely access CAN High (pin 6) and CAN Low (pin 14) at the diagnostic port without back-probing or piercing wires. Set the DSO to DC coupling, and probe CANH on one channel and CANL on a second channel. Trigger on either channel and capture several milliseconds of data.

You should see a clean square-wave pattern where CANH and CANL mirror each other. Measure peak voltage, recessive voltage, and compare to specification. A missing signal on one wire while the other wire is active points to a wiring open or short. A signal that does not return to recessive level suggests a short between CANH and CANL or a module holding the bus dominant (a condition called bus-off).

Safety

Key terms9

CAN Bus (Controller Area Network)A two-wire differential serial network standard that allows vehicle control modules to communicate with each other without a central host computer.
Differential SignalingA transmission method in which the meaningful signal is the voltage difference between two wires (CANH and CANL), which rejects common-mode electrical noise from either wire.
Recessive StateThe idle or logic-one condition of a CAN bus where both CANH and CANL sit near 2.5 V and the differential voltage is approximately 0 V.
Dominant StateThe active or logic-zero condition of a CAN bus where CANH rises to approximately 3.5 V and CANL drops to approximately 1.5 V, producing a differential of approximately 2 V.
U-CodeAn SAE J2012 diagnostic trouble code category for network communication faults; a U0xxx code typically indicates a module has lost communication with another module.
Gateway ModuleA module that bridges two or more CAN bus segments running at different speeds, translating messages so modules on separate segments can share data.
Bus-OffA fault condition in which a module or wiring fault holds the CAN bus in the dominant state continuously, preventing all other modules from transmitting.
Node AddressA unique identifier assigned to each module on a CAN network segment that distinguishes its messages from those of all other modules on the same bus.
Cascade FaultThe condition in which the failure of one network module causes multiple other modules to each store their own U-codes because they no longer receive expected data messages.

Worked examples4

1. A DSO is connected to CANH and CANL at the OBD-II port. CANH measures 3.5 V peak and 2.5 V recessive. CANL measures 1.5 V trough and 2.5 V recessive. Calculate the differential voltage in the dominant state.
  1. Identify CANH voltage in the dominant state: 3.5 V
  2. Identify CANL voltage in the dominant state: 1.5 V
  3. Subtract CANL from CANH: 3.5 V − 1.5 V = 2.0 V
  4. Compare to specification: the standard dominant-state differential for high-speed CAN is approximately 2 V — this reading is within spec.
Answer: Differential voltage in the dominant state = 2.0 V, which is within the expected specification for a high-speed CAN bus.
2. In the recessive state, CANH reads 2.6 V and CANL reads 2.4 V. Is this within the expected recessive differential of approximately 0 V?
  1. Identify CANH recessive voltage: 2.6 V
  2. Identify CANL recessive voltage: 2.4 V
  3. Calculate differential: 2.6 V − 2.4 V = 0.2 V
  4. Compare to specification: the recessive state differential should be near 0 V; 0.2 V is a small deviation likely within tolerance for many systems, but you should compare to the manufacturer's published specification and note the reading on your worksheet.
Answer: Recessive differential = 0.2 V. This is close to 0 V and likely acceptable, but always compare to the manufacturer's published specification for the specific vehicle.
3. After a full network scan, modules A, B, C, and D all set U-codes indicating lost communication with module E. Module E shows no response. Technician checks service information and finds modules A, B, C, D, and E all share the same high-speed CAN segment. What is the correct diagnostic starting point, and why?
  1. Recognize that modules A–D each stored a U-code only because they expected data from module E — this is a cascade fault pattern.
  2. Identify module E as the non-communicating root module because it is the only module that is not responding to the scan tool.
  3. Before condemning module E, verify power supply and ground circuits at module E's connector using a wiring diagram, because a blown fuse or bad ground can disable a module without any internal failure.
  4. Also check the CAN bus wiring between module E's connector and the main network to rule out a broken splice or connector corrosion.
Answer: Start diagnosis at module E — the non-responding module — by checking its power, ground, and CAN bus wiring before assuming the module itself has failed.
4. A CANH waveform shows a normal square wave. The CANL waveform is a flat line at 2.5 V with no activity. Calculate what the differential waveform would read in what should be the dominant state.
  1. In the dominant state, CANH should rise to approximately 3.5 V — assume it does because CANH looks normal.
  2. CANL is stuck at 2.5 V instead of dropping to 1.5 V.
  3. Calculate the differential: 3.5 V − 2.5 V = 1.0 V instead of the expected 2.0 V.
  4. Interpret the result: the differential signal is reduced by 50%. Many modules will fail to decode data correctly, and the most likely cause is an open circuit or short to a fixed voltage on the CANL wire.
Answer: Dominant-state differential = 1.0 V, half of specification. The flat CANL line indicates a wiring fault — likely an open or a short to approximately 2.5 V — on the CANL wire.
In class: Illustrated lecture on network architecture: CAN High/Low voltage levels, differential signaling, module addressing, gateway modules, and how a single module fault can cascade U-codes across multiple systems; students complete a guided diagram of a sample vehicle network map. · Scan tool network scan lab: students run a full network scan on an assigned vehicle, record all U-codes and non-communicating modules on a structured worksheet, then use service information to locate the affected modules and identify their network segment. · DSO CAN bus measurement: using a breakout adapter at the OBD-II port, students probe CAN High and CAN Low, capture the differential waveform at idle, and compare peak voltage, recessive voltage, and signal shape to published specifications, documenting findings on a waveform sheet.
Materials: Professional scan tool with network scan capability, DSO with CAN bus probing adapter, OBD-II breakout box, vehicle with seeded U-codes or network trainer, service information (network topology diagrams), wiring diagrams, U-code recording worksheet, safety glasses
Check: Students submit completed network scan worksheet with all U-codes correctly described and module locations identified, plus one annotated CAN waveform with voltage levels labeled and compared to spec. Instructor verifies waveform capture was live from vehicle/trainer. Pass mark: 75% accuracy on worksheet.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

A6 Electrical/Electronic Systems
1. In a CAN bus differential signal, what voltage levels are expected on CANH and CANL when the bus is in the dominant state on a high-speed network?
A6 Electrical/Electronic Systems
2. A technician performs a network scan and finds eight U-codes on eight different modules, all indicating lost communication with the BCM. The BCM shows no response on the scan tool. What is the MOST likely explanation?
A6 Electrical/Electronic Systems
3. What is the PRIMARY advantage of differential signaling on a CAN bus compared to a single-wire network?
A6 Electrical/Electronic Systems
4. When probing the OBD-II connector to measure CAN bus signals, which OBD-II pin numbers carry CAN High and CAN Low on a standard ISO 15765-4 compliant vehicle?
A6 Electrical/Electronic Systems
5. A DSO waveform shows that CANH displays a clean square wave, but CANL is a flat line at approximately 2.5 V with no waveform activity. Which fault is MOST consistent with this finding?
A6 Electrical/Electronic Systems
6. What is the function of a gateway module in a vehicle network system?
A6 Electrical/Electronic Systems
7. Which SAE diagnostic trouble code category is used to identify network communication faults between control modules?
A6 Electrical/Electronic Systems
8. Before handling Supplemental Restraint System (SRS) components while diagnosing a network fault near the airbag control module, what is the MOST important safety step?
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