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Home / DTC / U0012 — Medium Speed CAN Communication Bus (+) Open

U0012 — Medium Speed CAN Communication Bus (+) Open

Detailed page for trouble code U0012.

35,619codes
59brands
11,925generic
23,694specific
All codes P-codes (Powertrain) B-codes (Body) C-codes (Chassis) U-codes (Network/User) Tesla Alerts
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Code

U0012

Generic U — Network/User

Medium Speed CAN Communication Bus (+) Open

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

Causes

  • Open or high-resistance wiring on the medium-speed CAN positive conductor (broken wire, damaged insulation, pin pushed out)
  • Corroded, loose, or damaged connector or pin at a module or splice
  • Failed CAN transceiver or module internal driver on the CAN+ side
  • Missing or blown fuse or power/ground related fault affecting a node on the medium-speed bus
  • Incorrect or missing termination (damaged resistor or removed termination)
  • Aftermarket accessory or repair damage to the CAN harness

Symptoms

  • Loss of communication with one or more modules on the medium-speed CAN network
  • Multiple network-related DTCs or modules showing 'no response' in a scan tool
  • Nonfunctional systems that rely on medium-speed bus messages (examples vary by vehicle)
  • Intermittent operation of affected systems or warning lights
  • Possible MIL illumination

What to check

  • Use a scan tool to read active/frozen DTCs and check which modules report on the medium-speed bus
  • Visually inspect wiring harnesses, connectors, and splices along the medium-speed bus route for damage, corrosion, or loose pins
  • With key ON (engine off), backprobe CAN+ at suspect modules and measure voltage relative to chassis ground
  • Check continuity/resistance of CAN+ conductor between nodes with ignition off (isolate circuits/modules as needed)
  • Measure end-to-end bus termination resistance (expected ~60 ohms for two 120Ω terminators in parallel) between CAN+ and CAN-
  • Check for shorts to +B or ground on the CAN+ line

Signal parameters

  • Recessive state (idle): CAN+ (CAN_H) ~2.5 V (approx.) relative to chassis ground
  • Dominant state: CAN+ ~3.5 V and CAN- ~1.5 V (approx.), differential ~2.0 V
  • If single-wire medium-speed CAN is used (SW-CAN), idle ~2.5 V and dominant near ~0–1 V; check vehicle wiring specification
  • Bus termination: two 120 Ω terminators in parallel ≈ 60 Ω measured between CAN+ and CAN-
  • Open condition: CAN+ may read an unexpected voltage (floating, 0 V, or battery voltage) depending on failure

Diagnostic algorithm

  1. Use an OEM-capable scan tool. Record all active and history U-codes and the modules that are not responding on the medium-speed bus.
  2. Verify DTC is current: clear codes and perform ignition cycles/drives to see if U0012 returns.
  3. Visually inspect the medium-speed CAN harness, connectors, and any recent repair or accessory work for damage or disconnected plugs.
  4. With ignition ON (engine off), measure CAN+ voltage at multiple modules and compare to expected idle voltage (~2.5 V). Identify nodes with abnormal voltage.
  5. With ignition OFF, check continuity of the CAN+ conductor between modules and to splice points. Isolate sections by disconnecting modules to find the open.
  6. Measure termination resistance between CAN+ and CAN- (expect ≈60 Ω). Remove connectors as required to isolate and confirm termination integrity.
  7. Check for shorts to battery or ground on the CAN+ line using an ohmmeter or appropriate short-detection method.
  8. If a specific module is suspected (open at that connector), inspect/repair the connector pins and wiring; replace the module only after wiring is confirmed good.
  9. Repair or replace damaged wiring, connectors, or defective module transceiver as indicated. Reassemble and secure harnesses to prevent future failures.
  10. Clear DTCs, re-scan and perform a functional test/road test to confirm the bus communications are restored and the code does not return.

Likely causes

  • Physical break or intermittent open in CAN+ conductor between modules
  • Corroded connector or water intrusion at a network splice or module
  • One module with a failed CAN+ transceiver pulling the line open
  • Poor pin seating or bent pin at a module connector
  • Repair or retrofit work disturbed the medium-speed CAN wiring

Fault status

⚠️ Status
Medium Speed CAN (+) Open — an open or very high resistance was detected on the medium-speed CAN positive conductor, causing communication loss or degradation on that network.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-3.0 hours

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Code

U0012

HYUNDAI U — Network/User

Medium Speed CAN Communication Bus (+) Open

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

Causes

  • Open or high-resistance wiring on the medium-speed CAN positive conductor (broken wire, damaged insulation, pin pushed out)
  • Corroded, loose, or damaged connector or pin at a module or splice
  • Failed CAN transceiver or module internal driver on the CAN+ side
  • Missing or blown fuse or power/ground related fault affecting a node on the medium-speed bus
  • Incorrect or missing termination (damaged resistor or removed termination)
  • Aftermarket accessory or repair damage to the CAN harness

Symptoms

  • Loss of communication with one or more modules on the medium-speed CAN network
  • Multiple network-related DTCs or modules showing 'no response' in a scan tool
  • Nonfunctional systems that rely on medium-speed bus messages (examples vary by vehicle)
  • Intermittent operation of affected systems or warning lights
  • Possible MIL illumination

What to check

  • Use a scan tool to read active/frozen DTCs and check which modules report on the medium-speed bus
  • Visually inspect wiring harnesses, connectors, and splices along the medium-speed bus route for damage, corrosion, or loose pins
  • With key ON (engine off), backprobe CAN+ at suspect modules and measure voltage relative to chassis ground
  • Check continuity/resistance of CAN+ conductor between nodes with ignition off (isolate circuits/modules as needed)
  • Measure end-to-end bus termination resistance (expected ~60 ohms for two 120Ω terminators in parallel) between CAN+ and CAN-
  • Check for shorts to +B or ground on the CAN+ line

Signal parameters

  • Recessive state (idle): CAN+ (CAN_H) ~2.5 V (approx.) relative to chassis ground
  • Dominant state: CAN+ ~3.5 V and CAN- ~1.5 V (approx.), differential ~2.0 V
  • If single-wire medium-speed CAN is used (SW-CAN), idle ~2.5 V and dominant near ~0–1 V; check vehicle wiring specification
  • Bus termination: two 120 Ω terminators in parallel ≈ 60 Ω measured between CAN+ and CAN-
  • Open condition: CAN+ may read an unexpected voltage (floating, 0 V, or battery voltage) depending on failure

Diagnostic algorithm

  1. Use an OEM-capable scan tool. Record all active and history U-codes and the modules that are not responding on the medium-speed bus.
  2. Verify DTC is current: clear codes and perform ignition cycles/drives to see if U0012 returns.
  3. Visually inspect the medium-speed CAN harness, connectors, and any recent repair or accessory work for damage or disconnected plugs.
  4. With ignition ON (engine off), measure CAN+ voltage at multiple modules and compare to expected idle voltage (~2.5 V). Identify nodes with abnormal voltage.
  5. With ignition OFF, check continuity of the CAN+ conductor between modules and to splice points. Isolate sections by disconnecting modules to find the open.
  6. Measure termination resistance between CAN+ and CAN- (expect ≈60 Ω). Remove connectors as required to isolate and confirm termination integrity.
  7. Check for shorts to battery or ground on the CAN+ line using an ohmmeter or appropriate short-detection method.
  8. If a specific module is suspected (open at that connector), inspect/repair the connector pins and wiring; replace the module only after wiring is confirmed good.
  9. Repair or replace damaged wiring, connectors, or defective module transceiver as indicated. Reassemble and secure harnesses to prevent future failures.
  10. Clear DTCs, re-scan and perform a functional test/road test to confirm the bus communications are restored and the code does not return.

Likely causes

  • Physical break or intermittent open in CAN+ conductor between modules
  • Corroded connector or water intrusion at a network splice or module
  • One module with a failed CAN+ transceiver pulling the line open
  • Poor pin seating or bent pin at a module connector
  • Repair or retrofit work disturbed the medium-speed CAN wiring

Fault status

⚠️ Status
Medium Speed CAN (+) Open — an open or very high resistance was detected on the medium-speed CAN positive conductor, causing communication loss or degradation on that network.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-3.0 hours

Similar codes

U0010 U0073 U0100 U0101 U0121
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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+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
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Code

U0012

LAND ROVER U — Network/User

Medium speed CAN communication bus (+) open

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

Causes

  • Open or high-resistance wiring on the medium-speed CAN positive conductor (broken wire, damaged insulation, pin pushed out)
  • Corroded, loose, or damaged connector or pin at a module or splice
  • Failed CAN transceiver or module internal driver on the CAN+ side
  • Missing or blown fuse or power/ground related fault affecting a node on the medium-speed bus
  • Incorrect or missing termination (damaged resistor or removed termination)
  • Aftermarket accessory or repair damage to the CAN harness

Symptoms

  • Loss of communication with one or more modules on the medium-speed CAN network
  • Multiple network-related DTCs or modules showing 'no response' in a scan tool
  • Nonfunctional systems that rely on medium-speed bus messages (examples vary by vehicle)
  • Intermittent operation of affected systems or warning lights
  • Possible MIL illumination

What to check

  • Use a scan tool to read active/frozen DTCs and check which modules report on the medium-speed bus
  • Visually inspect wiring harnesses, connectors, and splices along the medium-speed bus route for damage, corrosion, or loose pins
  • With key ON (engine off), backprobe CAN+ at suspect modules and measure voltage relative to chassis ground
  • Check continuity/resistance of CAN+ conductor between nodes with ignition off (isolate circuits/modules as needed)
  • Measure end-to-end bus termination resistance (expected ~60 ohms for two 120Ω terminators in parallel) between CAN+ and CAN-
  • Check for shorts to +B or ground on the CAN+ line

Signal parameters

  • Recessive state (idle): CAN+ (CAN_H) ~2.5 V (approx.) relative to chassis ground
  • Dominant state: CAN+ ~3.5 V and CAN- ~1.5 V (approx.), differential ~2.0 V
  • If single-wire medium-speed CAN is used (SW-CAN), idle ~2.5 V and dominant near ~0–1 V; check vehicle wiring specification
  • Bus termination: two 120 Ω terminators in parallel ≈ 60 Ω measured between CAN+ and CAN-
  • Open condition: CAN+ may read an unexpected voltage (floating, 0 V, or battery voltage) depending on failure

Diagnostic algorithm

  1. Use an OEM-capable scan tool. Record all active and history U-codes and the modules that are not responding on the medium-speed bus.
  2. Verify DTC is current: clear codes and perform ignition cycles/drives to see if U0012 returns.
  3. Visually inspect the medium-speed CAN harness, connectors, and any recent repair or accessory work for damage or disconnected plugs.
  4. With ignition ON (engine off), measure CAN+ voltage at multiple modules and compare to expected idle voltage (~2.5 V). Identify nodes with abnormal voltage.
  5. With ignition OFF, check continuity of the CAN+ conductor between modules and to splice points. Isolate sections by disconnecting modules to find the open.
  6. Measure termination resistance between CAN+ and CAN- (expect ≈60 Ω). Remove connectors as required to isolate and confirm termination integrity.
  7. Check for shorts to battery or ground on the CAN+ line using an ohmmeter or appropriate short-detection method.
  8. If a specific module is suspected (open at that connector), inspect/repair the connector pins and wiring; replace the module only after wiring is confirmed good.
  9. Repair or replace damaged wiring, connectors, or defective module transceiver as indicated. Reassemble and secure harnesses to prevent future failures.
  10. Clear DTCs, re-scan and perform a functional test/road test to confirm the bus communications are restored and the code does not return.

Likely causes

  • Physical break or intermittent open in CAN+ conductor between modules
  • Corroded connector or water intrusion at a network splice or module
  • One module with a failed CAN+ transceiver pulling the line open
  • Poor pin seating or bent pin at a module connector
  • Repair or retrofit work disturbed the medium-speed CAN wiring

Fault status

⚠️ Status
Medium Speed CAN (+) Open — an open or very high resistance was detected on the medium-speed CAN positive conductor, causing communication loss or degradation on that network.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-3.0 hours

Similar codes

U0010 U0073 U0100 U0101 U0121
413

Browse 413 LAND ROVER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

LAND ROVER

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+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
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Code

U0012

MERCEDES-BENZ U — Network/User

Medium Speed CAN Communication Bus (+) open

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

Causes

  • Open or high-resistance wiring on the medium-speed CAN positive conductor (broken wire, damaged insulation, pin pushed out)
  • Corroded, loose, or damaged connector or pin at a module or splice
  • Failed CAN transceiver or module internal driver on the CAN+ side
  • Missing or blown fuse or power/ground related fault affecting a node on the medium-speed bus
  • Incorrect or missing termination (damaged resistor or removed termination)
  • Aftermarket accessory or repair damage to the CAN harness

Symptoms

  • Loss of communication with one or more modules on the medium-speed CAN network
  • Multiple network-related DTCs or modules showing 'no response' in a scan tool
  • Nonfunctional systems that rely on medium-speed bus messages (examples vary by vehicle)
  • Intermittent operation of affected systems or warning lights
  • Possible MIL illumination

What to check

  • Use a scan tool to read active/frozen DTCs and check which modules report on the medium-speed bus
  • Visually inspect wiring harnesses, connectors, and splices along the medium-speed bus route for damage, corrosion, or loose pins
  • With key ON (engine off), backprobe CAN+ at suspect modules and measure voltage relative to chassis ground
  • Check continuity/resistance of CAN+ conductor between nodes with ignition off (isolate circuits/modules as needed)
  • Measure end-to-end bus termination resistance (expected ~60 ohms for two 120Ω terminators in parallel) between CAN+ and CAN-
  • Check for shorts to +B or ground on the CAN+ line

Signal parameters

  • Recessive state (idle): CAN+ (CAN_H) ~2.5 V (approx.) relative to chassis ground
  • Dominant state: CAN+ ~3.5 V and CAN- ~1.5 V (approx.), differential ~2.0 V
  • If single-wire medium-speed CAN is used (SW-CAN), idle ~2.5 V and dominant near ~0–1 V; check vehicle wiring specification
  • Bus termination: two 120 Ω terminators in parallel ≈ 60 Ω measured between CAN+ and CAN-
  • Open condition: CAN+ may read an unexpected voltage (floating, 0 V, or battery voltage) depending on failure

Diagnostic algorithm

  1. Use an OEM-capable scan tool. Record all active and history U-codes and the modules that are not responding on the medium-speed bus.
  2. Verify DTC is current: clear codes and perform ignition cycles/drives to see if U0012 returns.
  3. Visually inspect the medium-speed CAN harness, connectors, and any recent repair or accessory work for damage or disconnected plugs.
  4. With ignition ON (engine off), measure CAN+ voltage at multiple modules and compare to expected idle voltage (~2.5 V). Identify nodes with abnormal voltage.
  5. With ignition OFF, check continuity of the CAN+ conductor between modules and to splice points. Isolate sections by disconnecting modules to find the open.
  6. Measure termination resistance between CAN+ and CAN- (expect ≈60 Ω). Remove connectors as required to isolate and confirm termination integrity.
  7. Check for shorts to battery or ground on the CAN+ line using an ohmmeter or appropriate short-detection method.
  8. If a specific module is suspected (open at that connector), inspect/repair the connector pins and wiring; replace the module only after wiring is confirmed good.
  9. Repair or replace damaged wiring, connectors, or defective module transceiver as indicated. Reassemble and secure harnesses to prevent future failures.
  10. Clear DTCs, re-scan and perform a functional test/road test to confirm the bus communications are restored and the code does not return.

Likely causes

  • Physical break or intermittent open in CAN+ conductor between modules
  • Corroded connector or water intrusion at a network splice or module
  • One module with a failed CAN+ transceiver pulling the line open
  • Poor pin seating or bent pin at a module connector
  • Repair or retrofit work disturbed the medium-speed CAN wiring

Fault status

⚠️ Status
Medium Speed CAN (+) Open — an open or very high resistance was detected on the medium-speed CAN positive conductor, causing communication loss or degradation on that network.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-3.0 hours

Similar codes

U0010 U0073 U0100 U0101 U0121
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

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

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