U0019

Code

Generic U — Network/User

Low Speed CAN Communication Bus

Brand: Generic

Type: U — Network/User

Views: UK: 39 EN: 206 RU: 63

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or short in Low-Speed CAN wiring (CAN_L or CAN_H where applicable)
High resistance or intermittent connection at a connector or splice
Missing or incorrect bus termination (open or shorted 120 Ω resistors)
Poor battery supply or low system voltage
Poor or corroded ground at a module or chassis
Faulty or powered-down control module on the LS-CAN bus

Symptoms

Multiple modules on the LS-CAN bus fail to communicate or report status
Illumination of MIL or network-related warning lamps
Loss or intermittent operation of body functions (doors, windows, climate, infotainment, BCM-controlled devices)
No response from one or more modules via scan tool
Erratic or intermittent accessory behavior tied to low-speed network

What to check

Use a scan tool to read stored/freeze-frame U-codes and to identify which modules on the low-speed bus respond
Check battery voltage and charge state (low battery can cause network errors)
Visually inspect connectors, module grounds, and wiring along the LS-CAN trunk and branches for damage, corrosion, or loose terminals
Perform a wiggle test of harnesses and connectors while monitoring bus status with the scan tool
Measure bus termination resistance across CAN_H to CAN_L (or across the single-wire bus to ground where applicable) with power off
Probe bus voltages with a digital multimeter and compare to expected idle values; if available use an oscilloscope to view CAN waveforms and confirm healthy recessive/dominant transitions

Signal parameters

Idle (recessive) voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (differential ≈ 0 V)
Dominant state voltages (typical): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
Termination: typically 120 Ω at each end of a two-wire CAN bus (≈60 Ω measured across CAN_H to CAN_L)
Low-speed CAN data rates typically lower than powertrain CAN (commonly in the 33–125 kbps range; varies by vehicle)
Healthy waveform shows clean recessive/dominant edges, correct bit timing and no sustained dominant condition

Diagnostic algorithm

Record all U- and B-/P-codes and freeze-frame data; note affected modules and symptoms.
Verify battery voltage ≥12.4 V with engine off and charging system healthy; recharge if required and re-check codes.
Using a scan tool, attempt to communicate with each module on the low-speed CAN bus; note any non-responding modules.
Visually inspect connectors, wiring, and module mounting locations for damage, moisture, or corrosion; repair obvious issues.
With ignition on, measure DC voltages at the CAN bus pins of accessible modules; compare to nominal idle voltages (~2.5 V).
With vehicle power off, measure resistance between CAN_H and CAN_L (or bus terminals) to confirm ~60 Ω across the bus (two 120 Ω terminators in parallel). If open or very high, suspect missing termination; if very low, suspect a short to ground or 12 V.
If available, use an oscilloscope to view CAN waveforms at multiple points on the bus. Look for missing recessive states (sustained dominant), excessive noise, or missing edges.
Isolate the bus by disconnecting branches or individual modules (one at a time) and check whether the fault clears to find the faulty segment or module.
Inspect and test grounds and power feeds to modules on the bus; repair any poor connections.
Remove or disable aftermarket devices connected to the LS-CAN bus and retest.
If a specific module is suspected, follow module-specific diagnostics (power/ground/inputs/outputs) and consider module replacement or reprogramming per manufacturer procedures.
After repair, clear the codes and verify permanent code does not return; road test and re-scan to confirm network stability.

Likely causes

Open/shorted or chafed low-speed CAN wire between modules or near connectors
Bad ground or insufficient battery voltage causing bus instability
Missing/failed termination resistor(s) at bus ends
Intermittent connector/corrosion at a module on the LS-CAN bus
Failed module pulling the bus into a dominant state

Fault status

⚠️ Status

Controller reports loss or corruption of messages on the Low-Speed CAN communication bus. The bus may be open, shorted, improperly terminated, or a module may be driving the bus into a fault state. Network-dependent features may be degraded or inoperative.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours