Code
U0281
Generic
U — Network/User
Lost Communication With Control Module (Missing CAN Message)
Views:
UK: 4
EN: 5
RU: 3
AI status
Completed
Completed
100%
Causes
- Failed or unpowered control module (module not transmitting)
- Open, short, or high resistance in CAN-H or CAN-L wiring
- Damaged connector or corrosion at module connector or inline splice
- Missing or failed bus termination resistor(s) (incorrect ~60 Ω across bus)
- Short to battery or ground on CAN lines
- Faulty ground or supply to one or more modules affecting bus operation
Symptoms
- MIL or dash warnings for systems tied to missing module
- Loss of functionality for the system controlled by the missing module (e.g., ABS, TCM, body control functions)
- Module not present or shown as offline on a scan tool
- Multiple communication U-codes may be stored
- Intermittent operation of affected features
- No periodically expected CAN messages visible on a scope or scan tool
What to check
- Scan tool: read U0281 and other U-codes; note freeze frame and timestamps
- Use scan tool to list present modules and identify which module is missing
- Visually inspect connectors and wiring for physical damage, corrosion, or loose pins at the missing module and at gateway junctions
- Measure battery voltage and key-on voltage at the affected module power and ground pins
- With key off, measure resistance between CAN-H and CAN-L (expect ~60 Ω for two 120 Ω terminators in parallel)
- With key on, measure DC voltages: CAN-H and CAN-L should sit near 2.5 V (recessive) — check at multiple points along the bus
Signal parameters
- Idle/recessive state: CAN-H ≈ 2.5 V, CAN-L ≈ 2.5 V (each), differential ≈ 0 V
- Dominant state: CAN-H ≈ 3.5 V, CAN-L ≈ 1.5 V, differential ≈ 2.0 V
- Bus termination: two 120 Ω terminators in parallel → ≈ 60 Ω across CAN-H and CAN-L with ignition off (or power disabled)
- Typical bus speeds: commonly 500 kb/s (high-speed) or 250 kb/s; low-speed CAN may be 125 kb/s or lower
- Module message rate: varies by module — may be periodic messages every 10–1000 ms depending on function
Diagnostic algorithm
- Record the exact code, freeze frame, and any related U-codes using a factory-level or advanced scan tool.
- Identify which module the messages are missing from and determine which physical CAN network (HS-CAN, MS-CAN, LS-CAN) it uses.
- Verify vehicle battery voltage and inspect module power and ground circuits for proper voltage and good grounds.
- Perform a visual inspection of the module connector, harness, and any intermediate junctions for damage, corrosion, or pin deformation. Repair any obvious issues.
- With ignition on, verify module presence on the scan tool. If the module is absent, check for fused power to the module and apply power if safe to test (observe manufacturer procedures).
- Measure DC voltages on CAN-H and CAN-L at the module connector and at a known-good point on the same bus. Expect ~2.5 V each when idle. Significant deviation indicates wiring or termination problem.
- With ignition off (power removed per vehicle procedure), measure resistance across CAN-H and CAN-L near each end of the bus; expected ≈60 Ω across the bus. A very high or very low reading suggests open or shorted termination.
- Check resistance from CAN lines to battery positive and chassis ground to locate shorts.
- Use an oscilloscope to inspect waveform integrity and timing; look for missing messages, noise, excessive reflections, or stuck dominant bits.
- Isolate sections of the bus if necessary by disconnecting modules or splicing to find the faulty segment.
- If wiring and bus health are confirmed, consider reflashing or replacing the missing module per factory procedure. After repair, clear codes and verify normal message presence and function.
- Re-test road/functional checks for affected systems and monitor for recurrence.
Likely causes
- Blown or corroded connector at the affected module
- Broken or pinched CAN wiring between modules or to gateway
- Failed module (no power, internal fault, or firmware crash)
- Bus termination missing or shorted causing messages to be unreadable
Fault status
Status
Lost Communication With Control Module — expected CAN messages from the control module are missing or not seen on the network. The module may be offline, its wiring may be open/shorted, or the CAN bus is impaired.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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