U1901

Code

Generic U — Network/User

Communication Bus 'A' Off (loss of network communication)

Brand: Generic

Type: U — Network/User

Views: UK: 19 EN: 63 RU: 21

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Multiple control modules not responding to scan tool
No network data messages or intermittent communications
Warning lights or MIL illuminated for multiple systems
Loss of instrument cluster data, engine/ABS/BCM features
Vehicle may go into limp mode or refuses to crank/start if critical modules are unreachable

Connect a diagnostic scan tool and confirm U1901 and any related U-codes; record freeze frame and module presence
Visually inspect wiring harnesses, connectors, and modules on Bus A for damage, corrosion, or water intrusion
Check fuses and power/ground circuits for modules that interface to Bus A
Measure DC voltage at CAN_H and CAN_L relative to chassis with ignition ON (expected ~2.5 V idle)
Measure resistance across CAN_H and CAN_L with ignition OFF — expected ~60 Ω (two 120 Ω terminators in parallel)
Check for short to battery or ground on each bus conductor with an ohmmeter

Idle (recessive) voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (measured vs chassis)
Dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
Termination: nominal two 120 Ω resistors (ends) → ~60 Ω across bus
Common data rates: 125 kbps, 250 kbps, 500 kbps (verify vehicle-specific rate)
No communication: bus may appear stuck dominant (one line low/high) or idle with no toggling

Confirm the code and note any accompanying U-codes; attempt to communicate with modules using a scan tool and list which modules respond.
Perform a visual inspection of Bus A wiring, connectors, and modules; check for recent repairs or aftermarket installs.
Verify power and ground supplies for gateway/interface modules and any modules that provide transceivers to Bus A; check related fuses.
With ignition ON, measure CAN_H and CAN_L voltages vs chassis. Look for stuck/high/low values or equal voltages indicating open/short conditions.
With ignition OFF, measure resistance between CAN_H and CAN_L at a convenient connector; expected ≈60 Ω. If much higher → open/poor termination; near 0 Ω → direct short.
Isolate by disconnecting sections or individual modules: unplug non-critical modules or aftermarket devices one at a time while watching bus resistance and scan tool communications to find the offending node.
Use an oscilloscope or dedicated CAN tester to view signal waveforms and confirm proper differential signaling and data activity. Look for collisions, constant dominant, or noise.
If a specific module is identified as faulty (holding bus dominant or not responding), verify power/ground and replace or reprogram module per manufacturer procedures. Repair or replace damaged wiring/terminators as required.
After repairs, clear codes and perform full system scan and road test to confirm normal communications and that U1901 does not return.