U0012

Code

Generic U — Network/User

Medium Speed CAN Communication Bus (+) Open

Brand: Generic

Type: U — Network/User

Views: UK: 31 EN: 55 RU: 33

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

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.
Verify DTC is current: clear codes and perform ignition cycles/drives to see if U0012 returns.
Visually inspect the medium-speed CAN harness, connectors, and any recent repair or accessory work for damage or disconnected plugs.
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.
With ignition OFF, check continuity of the CAN+ conductor between modules and to splice points. Isolate sections by disconnecting modules to find the open.
Measure termination resistance between CAN+ and CAN- (expect ≈60 Ω). Remove connectors as required to isolate and confirm termination integrity.
Check for shorts to battery or ground on the CAN+ line using an ohmmeter or appropriate short-detection method.
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.
Repair or replace damaged wiring, connectors, or defective module transceiver as indicated. Reassemble and secure harnesses to prevent future failures.
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