U0018

Code

Generic U — Network/User

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

Brand: Generic

Type: U — Network/User

Views: UK: 36 EN: 72 RU: 49

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

Retrieve and record all U-codes and related freeze-frame / snapshot data. Note which modules report errors.
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.
With ignition OFF, disconnect battery negative lead before performing resistance checks to avoid damaging ECU transceivers.
Measure DC resistance between CAN+ and CAN- at a convenient connector. Expect about 60 Ω (terminators). A very low resistance (
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.
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.
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.
Inspect and repair the damaged wire or connector. Replace corroded connectors or pigtails. Repair using proper automotive splice techniques and protective sleeving.
If the short remains after harness/connector repair, suspect internal module failure; bench-test or replace suspected module per manufacturer procedure.
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