U0021

Code

Generic U — Network/User

Low Speed CAN Communication Bus (+) Open

Brand: Generic

Type: U — Network/User

Views: UK: 36 EN: 71 RU: 34

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Broken or disconnected CAN + conductor (open circuit)
Corroded or loose connector/pin at a module or splice
Short/open at a splice or wiring harness damage (chafing, rodent damage)
Failed CAN transceiver or control module
Missing or damaged termination resistor(s)
Water intrusion or contamination in connectors

Symptoms

Communication-related DTCs (U-codes) and MIL illumination
Loss or intermittent operation of modules on the LS-CAN network (e.g., body, door, mirror, HVAC, instrument cluster functions)
No or garbled data from one or more modules when scanned
Systems that rely on the LS-CAN remain inoperative or default to backup mode

What to check

Use a scan tool to confirm active U0021 and any additional communication codes; record freeze-frame and module list
Visually inspect LS-CAN wiring, connectors, splices and modules for damage, corrosion, or water entry
Check module power and ground circuits feeding the CAN transceivers (fuses, battery feed, chassis grounds)
Measure termination resistance between CAN+ and CAN− with ignition off (expected ≈ 60 ohms for a properly terminated two-terminator bus)
Perform continuity check of CAN+ conductor between modules and to splice points (with harness connectors disconnected where practical)
Use an oscilloscope or CAN bus monitor to observe CAN+ and CAN− waveforms while cranking/ignition on

Signal parameters

Clean, healthy CAN bus: recessive state — both lines near mid-supply (~2.5 V); dominant state — CAN_H rises (~3.5 V) and CAN_L falls (~1.5 V), producing ~2 V differential
Termination resistance (two 120 Ω in parallel) measured across CAN+ and CAN− with key off: ≈ 60 Ω
Open on CAN+ typically shows no toggling on CAN+ line and an idle voltage pulled toward module input or open-circuit voltage
Oscilloscope shows valid CAN frames (differential pulses) when network is healthy; an open conductor will remove expected differential transitions

Diagnostic algorithm

Retrieve and record all active/pending codes and freeze-frame data. Note which modules are not responding on the scanner.
Perform a visual inspection of LS-CAN harness routing, connectors, and modules for obvious damage or corrosion. Repair any obvious faults.
Verify power and grounds to modules that interface to the LS-CAN (check fuses, fused links and chassis grounds). Repair as needed.
With ignition off, measure resistance between CAN+ and CAN− at an accessible connector to verify termination (~60 Ω).
Check continuity of the CAN+ conductor from the failing module connector back through splices to other network nodes. Repair any opens or high-resistance connections.
With ignition on, use a multimeter/oscilloscope to monitor CAN+ and CAN− signals while comparing to a known-good vehicle or known-good segment. Look for absence of toggling on CAN+ (open) or abnormal voltages.
Isolate the fault by disconnecting modules one at a time (where safe and practical) to see when the bus behavior or codes change—this can identify the segment or module causing the open.
If a module is suspected, inspect and repair the connector and wiring; if harness and connectors are good and the bus is still open, replace or repair the defective module or its CAN transceiver per manufacturer instructions.
After repairs, clear codes and verify network communications and affected systems return to normal during a test drive.
Safety note: take usual precautions when probing live circuits. If performing soldering or splicing, disconnect battery where required by vehicle manufacturer procedures.

Likely causes

Damaged wire or pin at a harness connector where the LS-CAN branch connects
Corroded connector or poor pin-to-socket contact at a network module
Open splice or broken conductor inside loom
Failed module transceiver on one end of the LS-CAN segment
Removed or burnt termination resistor causing loss of network reference

Fault status

⚠️ Status

Low Speed CAN bus positive conductor open — communication lost or intermittent on LS-CAN network. Inspect wiring, connectors, terminations, and module transceivers.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0 - 3.0 hours