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Home / DTC / U0018 — Medium Speed CAN Communication Bus (-) shorted to Bus (+)

U0018 — Medium Speed CAN Communication Bus (-) shorted to Bus (+)

Detailed page for trouble code U0018.

34,405codes
59brands
11,914generic
22,491specific
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Code

U0018

Generic U — Network/User

Medium Speed CAN Communication Bus (-) shorted to Bus (+)

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

Causes

  • Damaged or chafed wiring harness causing CAN- to contact CAN+
  • Corroded or pushed-pin connector creating an internal short
  • Water intrusion in connectors or junction blocks
  • Incorrect repair or wiring modification (pins swapped or bridged)
  • Failed control module with internal short between CAN terminals
  • Faulty or missing termination/resistor network

Symptoms

  • Multiple modules on the medium-speed CAN network report communication errors
  • Intermittent or permanent loss of functions that use the medium CAN network
  • Illumination of communication-related warning lamps
  • Modules in the network may be in a bus-off state or show multiple U-codes
  • Erratic operation of features controlled over the affected bus

What to check

  • Verify vehicle battery voltage is stable and >12 V
  • Read and record all stored U-codes and freeze-frame data
  • Visual inspection of CAN wiring, connectors, and junction blocks for damage, corrosion, or water
  • Measure resistance between CAN+ and CAN- with battery disconnected (expect ~60 ohms with proper termination)
  • Check continuity from each CAN conductor to chassis ground and to battery positive to identify unintended shorts
  • Use an oscilloscope to view CAN waveforms on CAN+ and CAN- while network is active

Signal parameters

  • Recessive state: CAN+ and CAN- near half supply (~2.5 V on 5 V systems) — both lines similar
  • Dominant state: CAN+ rises and CAN- falls relative to recessive; differential typically ~1–2 V
  • Proper termination: two 120 Ω resistors in parallel at network ends => ≈60 Ω between CAN+ and CAN- (with ignition off and battery disconnected)
  • If CAN- is shorted to CAN+: voltage difference between lines will be near 0 V and waveform will be lost or corrupted
  • If CAN- is shorted to battery positive: CAN- will sit near battery voltage and communication will be disrupted

Diagnostic algorithm

  1. Retrieve and record all U-codes and related freeze-frame / snapshot data. Note which modules report errors.
  2. Inspect visible CAN wiring and connectors for physical damage, pin push-out, corrosion, moisture or rodent damage, focusing on areas near body harness entry points and junction blocks.
  3. With ignition OFF, disconnect battery negative lead before performing resistance checks to avoid damaging ECU transceivers.
  4. Measure DC resistance between CAN+ and CAN- at a convenient connector. Expect about 60 Ω (terminators). A very low resistance (
  5. Measure resistance from each CAN conductor to battery positive and to ground. Any low resistance to battery positive indicates a short to power; to ground indicates a short to ground.
  6. Reconnect battery and use a scan tool to monitor live CAN bus data while watching voltages on CAN+ and CAN- with a high-impedance voltmeter or oscilloscope. Look for collapsed or identical voltages on both wires.
  7. If short suspected in harness, isolate sections by disconnecting modules/junctions one at a time and repeating resistance/voltage checks until the shorted segment is localized.
  8. Inspect and repair the damaged wire or connector. Replace corroded connectors or pigtails. Repair using proper automotive splice techniques and protective sleeving.
  9. If the short remains after harness/connector repair, suspect internal module failure; bench-test or replace suspected module per manufacturer procedure.
  10. After repair, clear codes, perform a key-on/self-test and verify the code does not return. Complete functional tests of affected systems and road test if required.

Likely causes

  • Physical wire damage at harness routing points (hinges, firewall, doors)
  • Connector corrosion at junctions or gateway module
  • Recent body or wiring repairs where CAN harness passes
  • Module failure where CAN transceiver has shorted internally

Fault status

⚠️ Status
Medium-speed CAN bus negative (CAN-) conductor is shorted to the bus positive (CAN+). Communication on the medium-speed CAN network is impaired; inspect wiring, connectors, and modules for shorts or faults.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-4 hours

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Code

U0018

HYUNDAI U — Network/User

Medium Speed CAN Communication Bus (-) shorted to Bus (+)

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

Causes

  • Damaged or chafed wiring harness causing CAN- to contact CAN+
  • Corroded or pushed-pin connector creating an internal short
  • Water intrusion in connectors or junction blocks
  • Incorrect repair or wiring modification (pins swapped or bridged)
  • Failed control module with internal short between CAN terminals
  • Faulty or missing termination/resistor network

Symptoms

  • Multiple modules on the medium-speed CAN network report communication errors
  • Intermittent or permanent loss of functions that use the medium CAN network
  • Illumination of communication-related warning lamps
  • Modules in the network may be in a bus-off state or show multiple U-codes
  • Erratic operation of features controlled over the affected bus

What to check

  • Verify vehicle battery voltage is stable and >12 V
  • Read and record all stored U-codes and freeze-frame data
  • Visual inspection of CAN wiring, connectors, and junction blocks for damage, corrosion, or water
  • Measure resistance between CAN+ and CAN- with battery disconnected (expect ~60 ohms with proper termination)
  • Check continuity from each CAN conductor to chassis ground and to battery positive to identify unintended shorts
  • Use an oscilloscope to view CAN waveforms on CAN+ and CAN- while network is active

Signal parameters

  • Recessive state: CAN+ and CAN- near half supply (~2.5 V on 5 V systems) — both lines similar
  • Dominant state: CAN+ rises and CAN- falls relative to recessive; differential typically ~1–2 V
  • Proper termination: two 120 Ω resistors in parallel at network ends => ≈60 Ω between CAN+ and CAN- (with ignition off and battery disconnected)
  • If CAN- is shorted to CAN+: voltage difference between lines will be near 0 V and waveform will be lost or corrupted
  • If CAN- is shorted to battery positive: CAN- will sit near battery voltage and communication will be disrupted

Diagnostic algorithm

  1. Retrieve and record all U-codes and related freeze-frame / snapshot data. Note which modules report errors.
  2. Inspect visible CAN wiring and connectors for physical damage, pin push-out, corrosion, moisture or rodent damage, focusing on areas near body harness entry points and junction blocks.
  3. With ignition OFF, disconnect battery negative lead before performing resistance checks to avoid damaging ECU transceivers.
  4. Measure DC resistance between CAN+ and CAN- at a convenient connector. Expect about 60 Ω (terminators). A very low resistance (
  5. Measure resistance from each CAN conductor to battery positive and to ground. Any low resistance to battery positive indicates a short to power; to ground indicates a short to ground.
  6. Reconnect battery and use a scan tool to monitor live CAN bus data while watching voltages on CAN+ and CAN- with a high-impedance voltmeter or oscilloscope. Look for collapsed or identical voltages on both wires.
  7. If short suspected in harness, isolate sections by disconnecting modules/junctions one at a time and repeating resistance/voltage checks until the shorted segment is localized.
  8. Inspect and repair the damaged wire or connector. Replace corroded connectors or pigtails. Repair using proper automotive splice techniques and protective sleeving.
  9. If the short remains after harness/connector repair, suspect internal module failure; bench-test or replace suspected module per manufacturer procedure.
  10. After repair, clear codes, perform a key-on/self-test and verify the code does not return. Complete functional tests of affected systems and road test if required.

Likely causes

  • Physical wire damage at harness routing points (hinges, firewall, doors)
  • Connector corrosion at junctions or gateway module
  • Recent body or wiring repairs where CAN harness passes
  • Module failure where CAN transceiver has shorted internally

Fault status

⚠️ Status
Medium-speed CAN bus negative (CAN-) conductor is shorted to the bus positive (CAN+). Communication on the medium-speed CAN network is impaired; inspect wiring, connectors, and modules for shorts or faults.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-4 hours

Similar codes

U0073 U0100 U0101 U0102 U0141 U0155
371

Browse 371 HYUNDAI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

HYUNDAI

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Code

U0018

LAND ROVER U — Network/User

Medium speed CAN communication bus (â €) short to dam (+)

Brand: LAND ROVER
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Damaged or chafed wiring harness causing CAN- to contact CAN+
  • Corroded or pushed-pin connector creating an internal short
  • Water intrusion in connectors or junction blocks
  • Incorrect repair or wiring modification (pins swapped or bridged)
  • Failed control module with internal short between CAN terminals
  • Faulty or missing termination/resistor network

Symptoms

  • Multiple modules on the medium-speed CAN network report communication errors
  • Intermittent or permanent loss of functions that use the medium CAN network
  • Illumination of communication-related warning lamps
  • Modules in the network may be in a bus-off state or show multiple U-codes
  • Erratic operation of features controlled over the affected bus

What to check

  • Verify vehicle battery voltage is stable and >12 V
  • Read and record all stored U-codes and freeze-frame data
  • Visual inspection of CAN wiring, connectors, and junction blocks for damage, corrosion, or water
  • Measure resistance between CAN+ and CAN- with battery disconnected (expect ~60 ohms with proper termination)
  • Check continuity from each CAN conductor to chassis ground and to battery positive to identify unintended shorts
  • Use an oscilloscope to view CAN waveforms on CAN+ and CAN- while network is active

Signal parameters

  • Recessive state: CAN+ and CAN- near half supply (~2.5 V on 5 V systems) — both lines similar
  • Dominant state: CAN+ rises and CAN- falls relative to recessive; differential typically ~1–2 V
  • Proper termination: two 120 Ω resistors in parallel at network ends => ≈60 Ω between CAN+ and CAN- (with ignition off and battery disconnected)
  • If CAN- is shorted to CAN+: voltage difference between lines will be near 0 V and waveform will be lost or corrupted
  • If CAN- is shorted to battery positive: CAN- will sit near battery voltage and communication will be disrupted

Diagnostic algorithm

  1. Retrieve and record all U-codes and related freeze-frame / snapshot data. Note which modules report errors.
  2. Inspect visible CAN wiring and connectors for physical damage, pin push-out, corrosion, moisture or rodent damage, focusing on areas near body harness entry points and junction blocks.
  3. With ignition OFF, disconnect battery negative lead before performing resistance checks to avoid damaging ECU transceivers.
  4. Measure DC resistance between CAN+ and CAN- at a convenient connector. Expect about 60 Ω (terminators). A very low resistance (
  5. Measure resistance from each CAN conductor to battery positive and to ground. Any low resistance to battery positive indicates a short to power; to ground indicates a short to ground.
  6. Reconnect battery and use a scan tool to monitor live CAN bus data while watching voltages on CAN+ and CAN- with a high-impedance voltmeter or oscilloscope. Look for collapsed or identical voltages on both wires.
  7. If short suspected in harness, isolate sections by disconnecting modules/junctions one at a time and repeating resistance/voltage checks until the shorted segment is localized.
  8. Inspect and repair the damaged wire or connector. Replace corroded connectors or pigtails. Repair using proper automotive splice techniques and protective sleeving.
  9. If the short remains after harness/connector repair, suspect internal module failure; bench-test or replace suspected module per manufacturer procedure.
  10. After repair, clear codes, perform a key-on/self-test and verify the code does not return. Complete functional tests of affected systems and road test if required.

Likely causes

  • Physical wire damage at harness routing points (hinges, firewall, doors)
  • Connector corrosion at junctions or gateway module
  • Recent body or wiring repairs where CAN harness passes
  • Module failure where CAN transceiver has shorted internally

Fault status

⚠️ Status
Medium-speed CAN bus negative (CAN-) conductor is shorted to the bus positive (CAN+). Communication on the medium-speed CAN network is impaired; inspect wiring, connectors, and modules for shorts or faults.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-4 hours

Similar codes

U0073 U0100 U0101 U0102 U0141 U0155
320

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Code

U0018

MERCEDES-BENZ U — Network/User

Medium Speed CAN Communication Bus (-) shorted to Bus (+)

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

Causes

  • Damaged or chafed wiring harness causing CAN- to contact CAN+
  • Corroded or pushed-pin connector creating an internal short
  • Water intrusion in connectors or junction blocks
  • Incorrect repair or wiring modification (pins swapped or bridged)
  • Failed control module with internal short between CAN terminals
  • Faulty or missing termination/resistor network

Symptoms

  • Multiple modules on the medium-speed CAN network report communication errors
  • Intermittent or permanent loss of functions that use the medium CAN network
  • Illumination of communication-related warning lamps
  • Modules in the network may be in a bus-off state or show multiple U-codes
  • Erratic operation of features controlled over the affected bus

What to check

  • Verify vehicle battery voltage is stable and >12 V
  • Read and record all stored U-codes and freeze-frame data
  • Visual inspection of CAN wiring, connectors, and junction blocks for damage, corrosion, or water
  • Measure resistance between CAN+ and CAN- with battery disconnected (expect ~60 ohms with proper termination)
  • Check continuity from each CAN conductor to chassis ground and to battery positive to identify unintended shorts
  • Use an oscilloscope to view CAN waveforms on CAN+ and CAN- while network is active

Signal parameters

  • Recessive state: CAN+ and CAN- near half supply (~2.5 V on 5 V systems) — both lines similar
  • Dominant state: CAN+ rises and CAN- falls relative to recessive; differential typically ~1–2 V
  • Proper termination: two 120 Ω resistors in parallel at network ends => ≈60 Ω between CAN+ and CAN- (with ignition off and battery disconnected)
  • If CAN- is shorted to CAN+: voltage difference between lines will be near 0 V and waveform will be lost or corrupted
  • If CAN- is shorted to battery positive: CAN- will sit near battery voltage and communication will be disrupted

Diagnostic algorithm

  1. Retrieve and record all U-codes and related freeze-frame / snapshot data. Note which modules report errors.
  2. Inspect visible CAN wiring and connectors for physical damage, pin push-out, corrosion, moisture or rodent damage, focusing on areas near body harness entry points and junction blocks.
  3. With ignition OFF, disconnect battery negative lead before performing resistance checks to avoid damaging ECU transceivers.
  4. Measure DC resistance between CAN+ and CAN- at a convenient connector. Expect about 60 Ω (terminators). A very low resistance (
  5. Measure resistance from each CAN conductor to battery positive and to ground. Any low resistance to battery positive indicates a short to power; to ground indicates a short to ground.
  6. Reconnect battery and use a scan tool to monitor live CAN bus data while watching voltages on CAN+ and CAN- with a high-impedance voltmeter or oscilloscope. Look for collapsed or identical voltages on both wires.
  7. If short suspected in harness, isolate sections by disconnecting modules/junctions one at a time and repeating resistance/voltage checks until the shorted segment is localized.
  8. Inspect and repair the damaged wire or connector. Replace corroded connectors or pigtails. Repair using proper automotive splice techniques and protective sleeving.
  9. If the short remains after harness/connector repair, suspect internal module failure; bench-test or replace suspected module per manufacturer procedure.
  10. After repair, clear codes, perform a key-on/self-test and verify the code does not return. Complete functional tests of affected systems and road test if required.

Likely causes

  • Physical wire damage at harness routing points (hinges, firewall, doors)
  • Connector corrosion at junctions or gateway module
  • Recent body or wiring repairs where CAN harness passes
  • Module failure where CAN transceiver has shorted internally

Fault status

⚠️ Status
Medium-speed CAN bus negative (CAN-) conductor is shorted to the bus positive (CAN+). Communication on the medium-speed CAN network is impaired; inspect wiring, connectors, and modules for shorts or faults.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1-4 hours

Similar codes

U0073 U0100 U0101 U0102 U0141 U0155
856

Browse 856 MERCEDES-BENZ manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

MERCEDES-BENZ

Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email

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