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DTC codes / U00xx · Network wiring and bus faults / U0004

U0004High Speed CAN Communication Bus (+) Low

U0004 means the vehicle's high-speed CAN (Controller Area Network) data bus, specifically the CAN+ wire, has been detected at an abnormally low voltage level. The computer that monitors the bus has seen the CAN+ signal fall below the expected idle or active threshold, which can disrupt communication between multiple electronic control modules.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaNetwork wiring and bus faults

Symptoms you may notice

  • Multiple warning lights illuminated simultaneously, often including the check engine light, ABS, and traction control
  • The engine may start and run normally with no drivability complaints, or it may crank but not start
  • Intermittent or complete failure of dashboard gauges, climate control, or infotainment functions
  • Modules such as the transmission control module or body control module may drop offline, causing erratic shifting or accessory malfunctions
  • A no-start condition if the bus fault prevents the engine control module from receiving necessary signals
  • Often no symptoms besides the warning light, especially if the fault is intermittent or confined to a non-critical segment

Common causes, most likely first

  1. A short to ground on the CAN+ wire, often from chafed insulation against a metal bracket or body panel
  2. A faulty control module internally shorting the CAN+ line low, pulling the entire bus voltage down
  3. Corroded or loose pins in a CAN bus connector, particularly at a module or splice pack
  4. Water intrusion into a wiring harness or connector causing corrosion and low resistance to ground
  5. Aftermarket accessories improperly tapped into the CAN+ circuit, such as alarm systems or telematics devices
  6. A damaged or pinched wire in a section of harness that flexes with engine or suspension movement
  7. A failed terminating resistor inside a module that alters bus bias and pulls CAN+ low

How to diagnose and fix it

  1. Connect a scan tool and record all stored codes from every module to identify which modules are offline and whether the fault is intermittent or permanent
  2. Measure resistance between CAN+ and ground with the battery disconnected; a reading well below 60 ohms suggests a short to ground or a failed module
  3. Visually inspect the CAN+ wiring at each module connector and along known chafe points for damage, corrosion, or aftermarket taps
  4. Disconnect modules one at a time and recheck bus voltage or resistance to isolate a module that is internally shorting the line
  5. Perform a voltage drop test on the CAN+ circuit while wiggling harness sections to catch intermittent shorts
  6. Repair or replace the damaged section of harness, connector pins, or the faulty module as indicated by the isolation tests
  7. Clear codes and verify communication on all modules with a scan tool, then road test to confirm the repair

When CAN+ Low Is Not a Wiring Short

  • A module that fails internally can pull CAN+ low without any visible wiring damage, so disconnecting modules one by one is often the fastest way to find the culprit.
  • Aftermarket remote starters, alarm systems, and GPS trackers are frequently tapped into the CAN bus and can introduce a low-resistance path that sets U0004 long after installation.
  • The code can be stored by a single module that loses communication only under specific conditions, such as vibration or temperature extremes, making it intermittent and hard to reproduce in the shop.
  • A weak or failing battery or charging system can cause modules to behave erratically and set communication codes, so always verify battery voltage and ground integrity before chasing the bus.
  • U0004 is a generic code, but some manufacturers use proprietary subcodes that pinpoint the exact module or wire segment, so check for manufacturer-specific codes with a factory-level scan tool.

Can I keep driving with U0004?

If the vehicle starts and runs normally with only a warning light, you can drive it to a repair shop, but avoid long trips and expect that communication failures could suddenly disable safety or drivetrain systems. If the engine cranks but does not start or multiple systems are inoperative, do not drive and have the vehicle towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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