U0039 — Vehicle Communication Bus B (+) Open

OBD2 code U0039 flags a problem with the vehicle network (CAN bus) communication (vehicle communication bus b (+) open). Short, gentle driving to a shop is usually possible. Common culprits: a faulty module pulling the bus down, poor grounds or low battery voltage, water ingress in harness junctions.

vehicle network

Is it safe to drive?

Drive with caution Depends on what is affected. A single dead convenience module is fine to drive with; loss of communication with engine, braking or steering modules is not. If multiple critical systems drop out, do not drive.

How to diagnose with an OBD scanner

Read the code and freeze frame Connect an OBD2 scanner, confirm U0039 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many U0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the vehicle network (CAN bus) communication for damage, corrosion, chafing or poor contact.
Subsystem check Measure resistance between CAN-High and CAN-Low at the OBD port with power off — about 60 Ω is healthy.
Subsystem check Scan all modules: the one that does not respond is either the culprit or downstream of the break.
Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm U0039 does not return.

Approximate repair cost

Repair cost depends on which component actually failed. Typical scenarios for U0039 (parts and labor, USD, independent shop rates): Remove interfering aftermarket device: $0–$150; Repair CAN wiring or connectors: $100–$400; Replace the faulty control module: $300–$1,500. Overall, fixing U0039 usually costs between $0 and $1,500. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Common questions

What does OBD2 code U0039 mean?

What is the most common cause of U0039?

The most common causes are: a faulty module pulling the bus down, poor grounds or low battery voltage, water ingress in harness junctions.

What are the symptoms of U0039?

Drivers most often notice: no-start or intermittent starting in some cases, modules intermittently not communicating with the scanner, random, hard-to-reproduce electrical glitches, intermittent loss of functions (gauges, windows, steering assist).

Is it safe to drive with U0039?

Depends on what is affected. A single dead convenience module is fine to drive with; loss of communication with engine, braking or steering modules is not. If multiple critical systems drop out, do not drive.

Will U0039 clear itself?

Sometimes U0039 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

What does U0039 mean?

Seeing U0039 on your scanner? The control unit has caught the vehicle network (CAN bus) communication operating outside its calibrated window. The formal description is “Vehicle Communication Bus B (+) Open”, and the code is grouped under vehicle network faults.

This code does not appear from a single bad reading: the module must see the fault condition repeat under defined operating conditions before it turns on the warning light. The freeze frame captured with U0039 — engine load, rpm, temperature, speed — tells you exactly when the problem occurs, so reproduce those conditions when testing.

Common symptoms of U0039

no-start or intermittent starting in some cases
modules intermittently not communicating with the scanner
random, hard-to-reproduce electrical glitches
intermittent loss of functions (gauges, windows, steering assist)
multiple warning lights on at once

What causes U0039?

aftermarket equipment interfering with the bus
water ingress in harness junctions
corroded connectors, especially in wet areas
a faulty module pulling the bus down
poor grounds or low battery voltage
damaged CAN bus wiring or short to power/ground

How to diagnose U0039

Connect an OBD2 scanner, confirm U0039 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many U0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
Visually inspect the wiring harness and connectors related to the vehicle network (CAN bus) communication for damage, corrosion, chafing or poor contact.
Measure resistance between CAN-High and CAN-Low at the OBD port with power off — about 60 Ω is healthy.
Scan all modules: the one that does not respond is either the culprit or downstream of the break.
Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm U0039 does not return.

How to fix U0039

Start with the cheapest and most common causes before replacing expensive parts:

Remove interfering aftermarket device — $0–$150
Repair CAN wiring or connectors — $100–$400
Replace the faulty control module — $300–$1,500

How much does it cost to fix U0039?

Repair cost depends on which component actually failed. Typical scenarios for U0039 (parts and labor, USD, independent shop rates):

Remove interfering aftermarket device: $0–$150
Repair CAN wiring or connectors: $100–$400
Replace the faulty control module: $300–$1,500

Overall, fixing U0039 usually costs between $0 and $1,500. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.