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Home / DTC / U0049 — Vehicle Communication Bus C (+) low

U0049 — Vehicle Communication Bus C (+) low

Detailed page for trouble code U0049.

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Code

U0049

MERCEDES-BENZ U — Network/User

Vehicle Communication Bus C (+) low

Brand: MERCEDES-BENZ
Type: U — Network/User
Views: UK: 21 EN: 22 RU: 25
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Page language: EN

Causes

  • Damaged or chafed CAN C (+) wiring or pin
  • Short to ground on the CAN C (+) conductor
  • Corroded or loose connector at a module or gateway
  • Faulty CAN transceiver (in one control module)
  • Open or incorrect bus termination resistor(s)
  • Low battery/poor battery connection or engine/ignition voltage issues

Symptoms

  • Multiple control modules on Bus C report communication errors or go offline
  • Dashboard warnings or master warning lamp(s) illuminate
  • Loss of vehicle features that use Bus C (may be intermittent)
  • Stored U0049 and related U0xx communication codes
  • Possible no-start or limp behavior if critical modules lose communication
  • Intermittent diagnostics or unpredictable module behavior

What to check

  • Connect a capable scan tool; read all modules and note which modules report on Bus C and any related codes
  • Perform a visual inspection of wiring harnesses, connectors and grounding points for damage, corrosion, or evidence of water intrusion near gateway and modules on Bus C
  • Check battery voltage and main ground connections (clean/tighten if needed)
  • With ignition ON (engine off), measure voltage at CAN C (+) and CAN C (−) at gateway/module connectors with a DMM and compare to expected values
  • Measure resistance across the bus (power off) to verify termination (expect ~60 ohms on a properly terminated CAN bus)
  • Use an oscilloscope to view CAN waveforms for proper recessive/dominant voltages and frame activity

Signal parameters

  • Normal recessive voltage: approx. 2.5 V on CAN C (+) and CAN C (−) each
  • Dominant state (typical): CAN C (+) ~3.5 V, CAN C (−) ~1.5 V (differential ≈ 2.0 V)
  • U0049 trigger: CAN C (+) measured significantly below recessive level (typically < ~2.0 V) or held near ground
  • Termination resistance (power off, across C+ to C−): ≈ 60 Ω (two 120 Ω terminators in parallel)
  • Short to ground: CAN C (+) near 0 V; short to battery: CAN C (+) near battery voltage/not expected waveform

Diagnostic algorithm

  1. Record all current and pending codes, and note which modules are present on Bus C using a scan tool.
  2. Verify battery voltage and main grounds; correct any low-voltage conditions before further testing.
  3. Visually inspect connectors, harnesses and known trouble spots along Bus C (gateway, junctions, modules). Repair obvious damage.
  4. With ignition ON (engine off), measure DC voltage at CAN C (+) and CAN C (−) at the gateway or central connector; compare to expected ≈2.5 V recessive. If C(+) is low, proceed to isolate.
  5. With power off, measure resistance across C+ to C− at the same point; expect ≈60 Ω. If open or significantly different, check termination resistors/line continuity.
  6. If DC measurements indicate a short, use back-probing or disconnect modules one at a time (per OEM safety procedure) to see when the low condition clears — this isolates the module or harness segment causing the fault.
  7. Use an oscilloscope on C+ and C− to verify proper CAN waveform timing and amplitude once low-voltage condition is resolved or to confirm which module affects the bus when reconnected.
  8. If a module transceiver is confirmed faulty, replace or repair the failing module per manufacturer procedure. Repair damaged wiring, connectors, or replace terminators as required.
  9. Clear codes, cycle ignition, and confirm proper operation and absence of U0049. Road-test and re-scan to ensure fault does not return.

Likely causes

  • Short to ground or low-voltage condition on the CAN C (+) conductor
  • Damaged connector or corrosion at a module/gateway on Bus C
  • Failed transceiver inside a module (gateway, ECU) pulling the line low
  • Missing or damaged termination resistor causing abnormal voltages

Fault status

⚠️ Status
Vehicle Communication Bus C (+) low — voltage on CAN Bus C positive circuit below expected threshold; communication on Bus C impaired or lost.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-4.0 hours

Similar codes

U0100 U0101 U0073 U0121 U0131
610

Browse 610 MERCEDES-BENZ manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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