U131B

Code

Generic U — Network/User

CAN Bus B Communication Fault

Brand: Generic

Type: U — Network/User

Views: UK: 12 EN: 6 RU: 2

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Page language: EN

Illumination of communication or network-related warning lamp(s)
Loss or intermittent operation of systems on CAN B (e.g., climate, body modules, infotainment, ADAS — vehicle dependent)
Stored U131B freeze frame or related U-codes in scan tool
Intermittent faults that clear or return with vibration or temperature change
Modules showing ‘no communication’ or incorrect data on a scan tool

Connect a diagnostic scan tool and read all network U-codes and module communication status
Record freeze-frame data and occurrence frequency
Visual inspection of CAN B wiring, connectors, and grounds for corrosion, damage, or loose pins
Verify battery voltage and main chassis/engine grounds are secure
Measure CAN bus termination resistance with ignition off (expect ~60 Ω across CAN_H and CAN_L)
Probe CAN_H and CAN_L with a DVM or preferably an oscilloscope with ignition on to verify idle voltages and waveform integrity

Idle (recessive) voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both rails)
Dominant state voltages: 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 (two 120 Ω terminators in parallel)
Common data rates: 125 kbps, 250 kbps, 500 kbps (vehicle-specific)
Allowable DMM voltage tolerance: ±0.3 V typical; scope waveform should show clean square/twisted differential transitions

Connect a scan tool and record active/pending U-codes and which modules on CAN B are non-responsive.
Verify battery voltage (>12 V) and inspect main ground straps; poor power/ground can mimic bus faults.
Visually inspect CAN B harness, connectors, and splices for corrosion, damage, or loose pins; repair any issues found.
With ignition ON, measure DC voltages at a known-good CAN connector: both CAN_H and CAN_L should be ≈2.5 V. If not, continue to step 6.
With ignition ON, measure resistance across CAN_H and CAN_L with ignition OFF; expect ≈60 Ω. If open or very high, check for missing/failed terminator(s).
Use an oscilloscope on CAN_H and CAN_L while exercising modules (or cranking if needed) to look for dominant/recessive states, noise, voltage spikes, and correct differential waveform. Identify modules causing waveform distortion.
If one module is suspected, disconnect that module’s connector (one at a time) while monitoring the bus to see if the fault clears — this can isolate a shorted or dominant device. Do not disconnect ECUs while powered if vehicle-specific procedures advise otherwise.
Repair or replace damaged wiring, connectors, or failed module transceivers as indicated. After repairs, clear codes and retest to confirm normal CAN B communication.
If wiring and hardware check out, verify module software versions and reflash/update modules per manufacturer procedures; then re-test communications.