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DTC codes / U00xx · Network wiring and bus faults / U0033

U0033Vehicle Communication Bus A (-) Open

U0033 means the vehicle's main communication network, known as Bus A, has an open circuit on its negative data line, so modules cannot exchange messages reliably. The computer detected that the voltage on the Bus A negative wire is outside the expected range, indicating a break or disconnection in that conductor.

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

Symptoms you may notice

  • Multiple warning lights illuminated at once, often including the check engine light, ABS, and airbag lights
  • The engine may start but run poorly or stall because the powertrain control module cannot communicate with other modules
  • Intermittent or complete failure of dashboard gauges, climate controls, or infotainment systems
  • The scan tool cannot communicate with some or all control modules on the network
  • Often no symptoms besides the warning light if the open is on a non-critical branch of the bus

Common causes, most likely first

  1. A broken or chafed wire in the Bus A negative circuit, often at a flex point or near a connector
  2. Corroded or loose pins in a communication bus connector
  3. A failed control module that is internally shorting or pulling the bus negative line down
  4. Damage from rodents chewing through wiring harness insulation
  5. Water intrusion into a connector or module causing corrosion and an open circuit
  6. Aftermarket accessories improperly spliced into the bus wiring
  7. A faulty termination resistor or splice pack within the bus network

How to diagnose and fix it

  1. Connect a scan tool and record which modules can and cannot communicate to narrow down which segment of the bus is affected
  2. Inspect the Bus A negative wire and its connectors for visible damage, corrosion, or loose pins, starting at the diagnostic link connector and working outward
  3. Measure resistance on the Bus A negative circuit from the diagnostic link connector to each module, looking for an open or excessively high resistance
  4. Perform a voltage drop test on the negative bus line while wiggling harnesses to catch intermittent opens
  5. Disconnect modules one at a time to see if the bus voltage returns to normal, isolating a faulty module
  6. Repair or replace the damaged section of wire or the faulty connector, then clear codes and verify communication with all modules
  7. If a module is found to be pulling the bus down, replace or reprogram that module as required

Why Bus A opens are rarely just a bad module

  • A single chafed wire in a door hinge boot or under the dash can take down the entire network, so always inspect harness routing before condemning a module.
  • The code can set intermittently from a connector that only loses contact over bumps, so a wiggle test while watching live bus voltage is more revealing than a static resistance check.
  • Aftermarket alarms, remote starters, or dash cams spliced into the bus wiring are a common hidden cause that many technicians overlook.
  • U0033 does not always mean the bus is dead; a high-resistance open on one branch can let some modules communicate while others drop off, mimicking multiple unrelated failures.
  • Rodent damage is more common than most drivers realize, especially in vehicles parked outdoors, and chewed insulation on the negative bus wire often goes unnoticed until communication fails.

Can I keep driving with U0033?

Do not drive the vehicle if multiple warning lights are on or the engine runs poorly, because the loss of communication can disable safety systems and cause stalling. If the only symptom is a warning light and the car drives normally, limit driving to a short trip to a repair shop and avoid highways.

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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