U024388

Code

VOLVO U — Network/User

CAN Communication Climate Subnet. Bus off

Brand: VOLVO

Type: U — Network/User

Views: UK: 17 EN: 23 RU: 25

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Page language: EN

Damaged or shorted CAN bus wiring (CAN_H/CAN_L) in the climate subnet
Corroded or loose connectors at HVAC module(s) or junction points
Failed CAN transceiver or internal fault in the climate control module
Missing, failed or incorrect termination resistor(s)
Another module on the subnet driving the bus incorrectly (short to power/ground)
Low or unstable vehicle battery/grounding problems

Read all stored U- and related codes from all modules and note freeze-frame data
Verify battery voltage and engine-off/engine-running supply (should be within specification)
Visually inspect HVAC harnesses, connectors, junctions and ground points for damage or corrosion
Check CAN_H to CAN_L resistance with power off (expected ~60 Ω with both terminations present)
Measure continuity to identify shorts to chassis ground or battery positive on CAN wires
Use scan tool to see which modules are present on the climate subnet and whether any report bus-off or error counters

Typical idle voltages (no dominant): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (common-mode)
Dominant state: CAN_H rises ~+1.0 V and CAN_L falls ~-1.0 V relative to idle (differential ~2 V)
Typical termination: two 120 Ω resistors in parallel = ~60 Ω between CAN_H and CAN_L
Common bitrates on vehicle networks: 250 kb/s or 500 kb/s (verify vehicle-specific bitrate before diagnosing)
Bus should show clean differential waveform without repeated error frames or prolonged dominant state

Capture all current faults and freeze-frame data with a full OBD2/diagnostic scanner; record which modules report presence/failure.
Clear codes and attempt to reproduce; note if U024388 returns immediately or intermittently.
Verify vehicle battery voltage and good chassis/body/engine grounds before further testing.
Inspect connectors and wiring for visible damage, water ingress or corrosion at HVAC module, junctions and CAN splices; repair as needed.
With ignition off, measure resistance between CAN_H and CAN_L at the climate module connector; confirm ~60 Ω. If open or high, check termination resistors.
Check for shorts: measure CAN_H and CAN_L to chassis ground and battery positive for low resistance (indicates short).
Use a lab scope or CAN bus analyzer with ignition on to observe bus traffic and error frames; look for repeated error frames, prolonged dominant states or a node driving bus incorrectly.
If scope shows a single node causing bus-off, isolate that module by disconnecting its connector and re-check bus behavior until the fault clears.
If a module is identified as misbehaving (transceiver fault), replace or reprogram per manufacturer procedure and re-test.
After repairs, clear codes, perform full system scan, operate HVAC functions and road-test to verify no recurrence. Document repair and re-run diagnostics.

⚠️ Status

CAN climate subnet entered bus-off state — communication to climate/HVAC modules interrupted.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5 - 4.0 hours

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Browse 221 VOLVO manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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