Car systems
CAN network and modules: how they work and what fails
A modern car has dozens of control modules (engine, transmission, ABS, airbags, body) that share information over data buses, mostly CAN. When one stops talking to the others, U codes appear along with faults that seem unrelated to each other.
Reviewed by Daniel Morales, founder of Reprorace SL, on
How it works
Each module reads its sensors, drives its actuators and publishes messages on the bus: speed, rpm or temperature reach the instrument cluster, the transmission or the stability control that way. There are usually several buses (powertrain, chassis, comfort, infotainment) joined by a gateway, which is also the door to the OBD port.
If a module does not receive the messages it expects, it stores a lost communication code (U0100 with the engine, U0121 with the ABS…); if the whole bus fails, bus codes appear (U0001). On newer cars, diagnostics can run over Ethernet (DoIP) and the gateway protects access to the write functions.
The most common faults
- A module's power supply: low voltage, a ground or a fuse: the module shuts down or resets and the others miss it.
- Bus wiring and connectors: moisture, chafing or a poor repair; the fault is usually intermittent.
- Termination resistors: when they fail, the bus works badly and errors appear in several modules.
- Faulty module: an internal fault (P0600 and following) or a programming one; sometimes a software update solves it.
- Poorly installed accessories: alarms, trackers or devices connected to the bus that disturb it.
A guide, most likely first: a repair shop confirms the diagnosis of each car with its own data.
Symptoms that can come from this system
Power windows or central locking failing: what it could be
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Instrument cluster goes dead or gauges fail: what to do
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ABS or ESP warning light on: what it means and what to do
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Key or immobilizer light: the car does not recognize the key
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Car radio or screen turns off by itself: why
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The most relevant trouble codes
- U0100Lost communication with ECM/PCM A
- U0001High speed CAN communication bus: malfunction
- U0101Lost communication with TCM
- U0121Lost communication with anti-lock brake system (ABS) control module A
- U0140Lost communication with body control module
- U0151Lost communication with restraints control module
- U0155Lost communication with instrument panel cluster (IPC) control module
- U0164Lost communication with HVAC control module
- U0401Invalid data received from ECM/PCM A
- U0073Control module communication bus A: bus off
- U0214Lost communication with remote function actuation module
- U0167Lost communication with vehicle immobilizer control module
- P0600Serial communication link: malfunction
- P0601Internal control module memory checksum: error
- P0606Control module: processor fault
Generic OBD-II/EOBD codes; each manufacturer adds its own. The same symptom can come from another system.
Code blocks of this system
- U01xxlost communication with control units
- U00xxcommunication bus (wiring and signals)
- P06xxcomputer and output circuits
- U03xxsoftware incompatibility between control modules
- U04xxinvalid data received from another control unit
- P26xxoutput signals, intake heater and fuel pump
- P08xxtransmission
- U02xxlost communication with other control modules
Questions and articles about this system
- What do U codes and lost communication (U0100) mean?
- Which diagnostic protocol does my car use: CAN, K-Line or DoIP?
- What is OBD-II and EOBD?
- Can an OBD reader damage the car or drain the battery?
- Can you program an ECU remotely, and what are the risks?
- Which diagnostic tool can I use remotely with ECO-LITE TOOL?
Learn more
See what your car is saying, system by system
The ECO-LITE plugs into the OBD port and shows the codes and live data your car publishes on your phone, explained; when needed, you share the session with your repair shop.
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Frequently asked questions
What does a U code mean?
That there is a communication problem between modules: it does not say which part is bad, but who stopped talking to whom. Start with the power supply and grounds of the module concerned.
Can reading the modules over OBD break anything?
Reading codes and data is a read-only operation. The risks lie in programming or coding, which needs a stable voltage and the manufacturer's procedures.
Why does my reader not see every module?
The generic part (OBD-II/EOBD) only requires what relates to emissions. The other modules use the manufacturer's diagnostics (UDS, KWP2000) and sometimes a gateway with protected access.