C0840

Code

Generic C — Chassis

Chassis CAN Bus: Message Missing from Module

Brand: Generic

Type: C — Chassis

Views: UK: 10 EN: 20 RU: 9

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Page language: EN

Illuminated dash warning lamps related to the affected system (ABS, traction, airbag, etc.)
Loss of the affected feature (e.g., ABS, steering assist, door functions)
Inability to communicate with the module using a diagnostic scanner
Intermittent communication faults or repeated C0840 appearance
Multiple modules reporting related communication errors

Read all network and module fault codes with a capable scan tool and note freeze frame/occurrence
Verify battery voltage and key-on power rails (12V) and check related fuses/relays
Visually inspect module connectors and harness for damage, corrosion, or pushed-out pins
Confirm the module is present in the vehicle’s network topology via scanner (attempt to communicate)
Check for other modules reporting bus faults (bus-off, uplink errors)
Measure CAN bus idle voltages at a known-good connector (CANH and CANL) with key on, engine off

CAN nominal idle voltage: CANH ≈ 2.5 V, CANL ≈ 2.5 V (recessive)
Dominant state: CANH ≈ 3.5 V, CANL ≈ 1.5 V (differential ≈ 2.0 V)
Differential voltage when active: ≈ 1.5–2.5 V depending on bus speed
Typical bus bit rates: 125 kb/s, 250 kb/s, or 500 kb/s (verify vehicle spec)
Termination resistance (end-to-end): ≈ 60 ohms (two 120 ohm resistors in parallel)

Capture and record all codes and freeze-frame data. Note which module/message ID is reported missing.
Confirm vehicle battery voltage is nominal and related fuses/relays that power the module are intact.
Use the scan tool to attempt communication with the specific module. If communication is successful, re-check for the message and clear codes to re-test.
If module does not respond, inspect and wiggle the module connector while monitoring for communication or intermittent codes.
Measure CANH and CANL voltages at the module connector with key on. Compare to expected idle voltages listed above.
Check termination resistance across CANH–CANL at the bus (approx. 60 ohms). If open or out-of-spec, trace wiring to locate broken/missing terminator.
Use an oscilloscope to view the CAN waveform at the module connector if available. Look for valid dominant/recessive levels, noise, missing frames, or bus contention.
If a short to battery or ground is detected on a CAN line, isolate by disconnecting sections or modules until the short location is found and repaired.
If wiring and termination are good but module still does not transmit, verify module supply and ground pins, then consider module replacement or reflashing per manufacturer procedure.
After repairs, clear codes, confirm normal communication and operation of the affected system, and perform a road test to confirm the fault does not return.