U0028

Code

Generic U — Network/User

Vehicle Communication Bus A

Brand: Generic

Type: U — Network/User

Views: UK: 38 EN: 108 RU: 45

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open, short, or high-resistance wiring on CAN Bus A (CAN_H/CAN_L)
Faulty or intermittent connector(s) at a module(s) or splice
Failed module placing noise or loading on the bus
Missing or incorrect termination (open or shorted terminators)
Low battery voltage or poor battery/ground connections
Blown fuse or power supply interruption to a module or gateway

Symptoms

Multiple modules reporting communication-related DTCs or missing module messages
Instrument cluster warnings or MIL illuminated
Loss of function for systems that communicate on Bus A (e.g., ABS, transmission, body modules)
Inability for the scan tool to communicate with some modules on Bus A
Intermittent malfunctions that change with vibration or temperature

What to check

Read all U-codes and module-specific communication codes with a capable scan tool; record freeze-frame and which modules report errors
Verify battery voltage and good ground connections before testing (battery charged, >12.4 V at rest)
Inspect fuses and power feeds for network gateway and affected modules
Visually inspect CAN Bus A wiring, connectors, and splice points for damage, corrosion, or pin push-outs
Measure DC voltages at suspected module power and ground pins
With ignition ON (engine off), measure CAN_H and CAN_L voltages to ground and resistance across CAN_H-L with ignition off (termination check)

Signal parameters

Idle (recessive) voltage to ground: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V
Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
Termination resistance: ~60 Ω measured across CAN_H to CAN_L with ignition OFF (two 120 Ω terminators in parallel)
Typical bus speeds: 125 kb/s, 250 kb/s, or 500 kb/s (verify vehicle specification)
Oscilloscope should show clean differential frames with proper recessive/dominant transitions; noise, stuck levels, or missing frames indicate a fault

Diagnostic algorithm

Capture and note all communication-related DTCs and freeze-frame data from every module. Do not clear codes yet.
Verify vehicle battery charged and good ground connections. Re-check codes after ensuring proper battery voltage.
Visually inspect wiring harnesses, connectors, and common routing near recent service areas or known wear points. Repair obvious damage.
With ignition ON (engine off), use a scan tool to see which modules are responsive on Bus A. Identify modules that are not communicating.
Measure CAN_H and CAN_L voltages to ground at a convenient connector with ignition ON. Confirm values near 2.5 V idle. If voltages are stuck high/low, isolate wiring or module causing the offset.
With ignition OFF, measure resistance across CAN_H to CAN_L at a convenient connector. Expect ~60 Ω. Open or very low resistance indicates missing or shorted terminator(s) or short to ground/power.
Use a scope to observe differential waveform while key on/engine cranking. Look for excessive noise, missing frames, or a dominant-stuck line. If available, check at both ends of the bus.
If a single module is suspected, disconnect that module (if safe to do so) and re-check bus behavior. If disconnecting the module clears the bus fault, repair or replace the module/wiring.
If fault persists across the bus, systematically isolate segments (disconnect branch connectors or bus repeaters/gateways) to locate the affected section.
Repair damaged wiring, replace failed terminators, and repair corroded connectors. Reprogram or replace modules only after wiring and power issues are resolved.
After repairs, clear codes, perform a functional test and re-check for reappearance of U0028 and any linked codes under normal operating conditions.

Likely causes

Broken or corroded CAN wiring or connector at a common splice or harness rub-through
Intermittent module failure that loads the bus (ECU with internal short)
Missing/failed termination resistor (open or shorted 120 Ω terminator)
Low battery/poor ground causing intermittent module resets

Fault status

⚠️ Status

Communication fault detected on Vehicle Communication Bus A. Messages are missing, invalid, or the bus is offline. Inspect CAN_H/CAN_L wiring, termination, power/ground, and module responses.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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