U1921

Code

Generic U — Network/User

Manufacturer-Specific Data Link Error

Brand: Generic

Type: U — Network/User

Views: UK: 15 EN: 14 RU: 14

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or short in data bus wiring
Poor connector or ground connection
Failed or intermittent control module
Incorrect or missing network termination (resistors)
Low battery or voltage supply issues
Software or firmware incompatibility/corruption

Symptoms

Intermittent or permanent communication faults reported by one or more modules
Multiple unrelated DTCs across modules (communication errors)
Loss of function for systems that rely on network data (e.g., instrument cluster, ABS, transmission)
Warning lights or messages on dash
Erratic module behavior or features not responding

What to check

Connect a factory-capable scan tool and read all stored/active U-codes from all modules
Inspect battery voltage and charging system (should be stable at 12–14.5V during checks)
Visually inspect wiring harnesses, connectors, and grounds related to affected modules and gateway
Check bus termination resistances (typical CAN bus ~60 ohms between CAN_H and CAN_L with key off)
Use bus monitor or oscilloscope to capture CAN/CAN FD/LIN bus activity and error frames
Check for recent repairs, module replacements, or aftermarket installations

Signal parameters

CAN bus typical data rates: 125 kbps, 250 kbps, 500 kbps or higher (verify vehicle-specific rate)
Idle (recessive) voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each relative to ground)
Dominant state differential: CAN_H ~3.5 V, CAN_L ~1.5 V (differential ≈ 2 V)
Termination resistance: ~60 ohms measured between CAN_H and CAN_L across the network
Error frame frequency: look for repeated error frames or bus-off states on a node
LIN/K-Line: check expected idle voltages and wake patterns if network uses those buses

Diagnostic algorithm

Retrieve and record all DTCs and freeze frame data from all modules with a full-capability scan tool.
Note if U1921 appears in multiple modules; if so, suspect a shared bus or gateway issue.
Verify battery and charging system voltage. Repair charging/battery issues first.
Visually inspect harness, connectors, and grounds for the modules on the affected bus (look for damage, corrosion, water).
Check termination resistance between CAN_H and CAN_L (engine off). Expect ~60 ohms; significantly different values indicate missing/shorted terminator.
Connect a CAN bus monitor or oscilloscope and observe CAN_H/CAN_L waveforms with key on and while driving or exercising the affected systems. Look for missing frames, repeated error frames, or bus-off events.
Perform wiggle/strain tests of harness and connectors while monitoring bus traffic to reproduce the fault and locate intermittent wiring faults.
Isolate the fault by disconnecting nonessential modules (one at a time) from the bus while monitoring for disappearance of the error to identify a bad node.
If a suspect module is identified, verify supply and ground circuits, inspect module connectors and pins, and update or reflash module software if available and recommended by OEM.
Repair any wiring, connector issues, or replace defective modules as indicated. Reinstall proper termination resistors if missing or damaged.
After repairs, clear codes and perform road test/functional test while monitoring bus traffic to confirm the fault does not return.
If intermittent and unresolved, consult manufacturer technical bulletins or wiring diagrams for network gateway configurations and known issues.

Likely causes

Damaged CAN high/low wires (chafing, pinched, broken)
Corroded or loose multi-pin connector at a gateway or module
Faulty gateway or communication module
Missing/bad 60 ohm total termination (two 120 ohm resistors) on CAN bus
Weak battery or failing charging system causing low bus voltage
Recent module replacement or incomplete programming

Fault status

⚠️ Status

Manufacturer-specific data link/network communication error. One or more modules report failure or intermittent loss of communication on the vehicle communication bus. Investigation of physical layer (wiring, connectors, termination), module supply/grounds, and bus traffic is required.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

Repair manuals