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Home / DTC / U0022 — Low Speed CAN Communication Bus (+) low

U0022 — Low Speed CAN Communication Bus (+) low

Detailed page for trouble code U0022.

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59brands
11,925generic
22,556specific
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Code

U0022

MERCEDES-BENZ U — Network/User

Low Speed CAN Communication Bus (+) low

Brand: MERCEDES-BENZ
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Short to ground on the CAN (+) conductor
  • Open or high-resistance connection in CAN (+) wiring or connector
  • Corroded or damaged CAN connector or pins
  • Failed CAN transceiver in one of the network modules
  • Missing or incorrect termination (open or shorted terminator)
  • Battery or chassis ground issue affecting CAN reference

Symptoms

  • Multiple modules on the low-speed CAN network show communication errors or fail to respond
  • Malfunctions of convenience systems (e.g., door modules, body control, instrument cluster) or warning lights
  • Stored related U-codes for modules on the same bus
  • Intermittent operation of low-speed network features
  • No or low activity shown on low-speed CAN when monitored with a scan tool or scope

What to check

  • Read and record all stored U-codes and module status with a capable scan tool
  • Perform a visual inspection of low-speed CAN wiring, connectors, and module grounds for damage, corrosion, or moisture
  • With key ON (engine off) measure CAN+ and CAN− static voltages at a connector using a digital multimeter
  • Measure resistance across CAN+ to CAN− with power off — check for approximate termination resistance (~60 ohms total)
  • Backprobe suspected module connector pins while monitoring bus with an oscilloscope or CAN-logging scan tool to view waveforms
  • Isolate sections by disconnecting modules one at a time (where safe) to find which node causes the voltage to go low

Signal parameters

  • Idle (recessive) — CAN+ and CAN− common-mode ≈ 2.5 V (both lines near the same voltage)
  • Dominant state — CAN+ typically rises and CAN− falls producing a differential ≈ 2.0 V (CAN+ ≈ 3.5 V, CAN− ≈ 1.5 V) depending on transceiver
  • Static fault condition — CAN+ stuck low below ~1.5–2.0 V or shows no dominant/recessive transitions
  • Termination — total bus resistance between CAN+ and CAN− ≈ 60 Ω (two 120 Ω terminators in parallel on many networks)
  • Message traffic — periodic data frames at the low-speed network bitrate visible on an oscilloscope or CAN logger

Diagnostic algorithm

  1. Record all DTCs and freeze/frame data; note which modules report communication loss.
  2. Inspect connectors, harnesses and module grounds for corrosion, damage, chafing or water ingress; repair obvious damage.
  3. With ignition ON (engine off) measure DC voltages: verify CAN+ and CAN− common-mode voltage and compare to expected ranges.
  4. With power off, measure resistance between CAN+ and CAN− to confirm termination (~60 Ω); also check CAN+ to ground and CAN− to ground for shorts or low resistance.
  5. Use an oscilloscope or CAN-capable scan tool to observe waveforms while the network is active; look for missing traffic, flat lines, or abnormal levels on CAN+.
  6. If a short or low voltage is present, isolate by disconnecting modules one at a time (starting with modules on the low-speed bus) until the fault clears to identify the offending node or harness section.
  7. If isolating points to a specific module, verify its supply and ground pins; replace the module only after confirming it is the root cause.
  8. Repair or replace damaged wiring, connectors, or terminators; clean corroded pins and apply dielectric grease as appropriate.
  9. After repairs, clear codes and verify normal communication and stable CAN waveforms; road test and re-scan for reoccurrence.

Likely causes

  • Wiring shorted to chassis ground or power rail on CAN+
  • Connector corrosion or bent pin at a gateway/module on the low-speed bus
  • Failed module transceiver pulling CAN+ low
  • Faulty termination resistor (open/short) causing incorrect bus voltage

Fault status

⚠️ Status
Low Speed CAN (+) voltage below expected threshold — low-speed network communication degraded or absent.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-3.0 hours

Similar codes

U0073 U0100 U0101 U0155 U0077
856

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