U0029

Code

Generic U — Network/User

Vehicle Communication Bus A Performance

Brand: Generic

Type: U — Network/User

Views: UK: 26 EN: 40 RU: 33

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open, short, or high-resistance wiring on CAN A (CANH/CANL)
Failed or intermittent module transceiver
Missing or failed termination resistor(s)
Corroded/loose connector or grounding issue
Aftermarket device or shorted accessory loading the bus
Intermittent power or battery voltage problems affecting modules

Symptoms

Multiple communication-related DTCs or modules reporting 'no communication' or 'restricted communication'
Illumination of MIL or multiple warning lamps
Loss or intermittent operation of systems controlled via CAN A (ABS, traction, transmission, body functions, etc.)
Modules present in scan tool intermittently or not at all
Intermittent/erratic vehicle behavior tied to a specific subsystem

What to check

Scan all modules and record all U-codes and any related P/C/B codes
Visually inspect CAN A wiring, connectors and grounds for damage, corrosion, or contamination
Measure CAN bus resistance between CANH and CANL with ignition off (expected ≈ 60 Ω with two 120 Ω terminators)
Measure DC voltages on CANH and CANL with key on (recessive ≈ 2.5 V each; verify values are within manufacturer spec)
Use an oscilloscope to view CANH/CANL waveforms for noise, missing dominant/recessive levels or incorrect bit timing
Disconnect aftermarket accessories on the bus (alarm, audio, telematics) and recheck

Signal parameters

Recessive: CANH ≈ 2.5 V, CANL ≈ 2.5 V (both lines near mid-supply)
Dominant: CANH ≈ 3.5 V, CANL ≈ 1.5 V (approx. 2 V differential)
Typical differential amplitude: ≈ 2.0 V
Bus resistance (two terminators in parallel): ≈ 60 Ω (with ignition off and both terminators connected)
Common bus bit rates: 125 kb/s, 250 kb/s, 500 kb/s (verify vehicle-specific baud rate)

Diagnostic algorithm

Record all active and stored codes from all modules; note time/date and any freeze-frame data.
Perform a visual inspection of CAN A wiring, connectors and module power/ground points for damage, moisture or loose pins.
With ignition off, measure resistance between CANH and CANL at a convenient connector; expect ≈ 60 Ω (two 120 Ω terminators).
With ignition ON (engine off), measure DC voltage on CANH and CANL relative to chassis ground to confirm recessive voltages (~2.5 V).
Use an oscilloscope to view CANH and CANL while communicating with modules. Look for proper dominant/recessive levels, clean edges, correct bit timing and absence of excessive noise or stuck dominant bits.
If bus voltage or waveform is out of spec, disconnect suspected modules (or remove aftermarket devices) one at a time and recheck to isolate the faulty module or wiring section.
Repair/replace damaged wiring, connectors, transceivers or termination as required. Replace a faulty control module only after isolating it as the cause.
Clear codes and re-test under conditions that reproduced the fault. Monitor for reoccurrence and verify all affected systems restore normal communication.

Likely causes

Loose/corroded connector at a control module on CAN A
Short to battery or ground on CANH or CANL from damaged wiring
Failed CAN transceiver in a single module pulling the bus out of spec
Missing 120 Ω termination or failed resistor causing improper bus impedance
Aftermarket audio, alarm or telematics device interfering with the bus

Fault status

⚠️ Status

Communication on Vehicle Communication Bus A is outside expected performance parameters. Messages are missing or corrupted, causing faults in one or more control modules.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0 - 3.0 hours