U0039 — Vehicle area network (VAN) data bus B (+) – open circuit #

Please note: The wording for this code may differ depending on your vehicle’s manufacturer.
Known label variations:

• U0039 – VAN Data Bus B (+) Open Circuit (General, Ford, Peugeot, Citroën, Renault)
• U0039 – Communication Line B (+) Open (GM, Opel/Vauxhall)
• U0039 – CAN/VAN Bus B (+) High Open (Mercedes-Benz)
• U0039 – Network Communication Bus B Circuit Open (Toyota, Honda, Nissan)

Summary of code U0039 #

Diagnostic Trouble Code (DTC) U0039 is related to the Vehicle Area Network (VAN) or similar data bus systems used for communication between different modules in your car. This specific code signals an open circuit on the Bus B (+) line, meaning the electrical connection is interrupted and data can’t flow as it should. Modern vehicles use these data buses—think of them as the nervous system of your car—to share information between modules like the powertrain control module (PCM), body control module (BCM), and others. When Bus B (+) is “open,” communication is lost, which can cause a cascade of electrical hiccups or failures. The result? Some vehicle functions may work intermittently or not at all, depending on how critical that bus is for your model.

Common symptoms for code U0039 #

When a U0039 code appears, your vehicle might display a range of symptoms. These can be subtle or quite obvious, depending on which control modules are affected by the loss of communication. Since the VAN bus is responsible for relaying vital information, you may experience anything from warning lights to non-functioning features. Look out for:

• Warning lights on the dashboard (ABS, airbag, check engine, etc.)
• Electronic systems failing or behaving erratically (power windows, climate control, infotainment, central locking)
• No communication with certain modules when scanned with a diagnostic tool
• Intermittent loss of function in connected systems
• Complete failure of a system such as steering assist, transmission, or lighting (rare but possible)

Probable causes for code U0039 (in order of frequency) #

A U0039 code almost always points to an issue with the physical wiring or connections in the VAN data bus B (+) circuit. However, sometimes the culprit lies deeper within a module or is even due to corrosion. Here are the most common causes:

1. Broken, damaged, or disconnected wiring in the VAN Bus B (+) circuit
2. Loose or corroded connectors at control modules or junction points
3. Faulty or shorted control module (rare, but possible if internal circuitry fails)
4. Water ingress or corrosion at connectors or under the hood
5. Recent repairs or aftermarket installations disturbing the harness

Step-by-step diagnostic for code U0039 #

Pinpointing a U0039 code requires careful inspection and the right approach. Rushing or skipping steps can lead to misdiagnosis. Here’s a structured method to diagnose the fault:

• 1. Scan all modules with a professional-level scan tool to identify modules not communicating or showing related codes.
• 2. Visually inspect the harness and connectors relevant to the VAN Bus B (+) circuit. Look for obvious damage, pinched wires, or disconnected plugs.
• 3. Check for water or corrosion at all accessible connectors, especially in the footwells, under the dash, and near the fusebox.
• 4. Perform a continuity test on the Bus B (+) wire using a multimeter to confirm the circuit is intact from end to end.
• 5. Wiggle test the harness while monitoring the scan tool to see if communication is restored or interrupted, indicating a poor connection.
• 6. If wiring is intact, isolate and test each module connected to the bus, disconnecting one at a time to see if the circuit recovers (which may indicate a faulty module).
• 7. Reference wiring diagrams from the OEM service manual for exact bus locations and module connections.

Possible repairs for code U0039 #

The fix for a U0039 code always depends on what you discover during diagnosis. Focus on the most probable causes first, and don’t overlook the basics. Here are the typical repairs:

• Repair or replace damaged wiring in the VAN Bus B (+) circuit
• Clean or replace corroded connectors at affected modules or junctions
• Secure or reconnect loose plugs after confirming they’re properly seated
• Replace faulty control modules only after confirming all wiring and power/ground circuits are good
• Seal and protect connectors from future water ingress or corrosion

Can you drive with code U0039? #

Limited. While your vehicle may still be drivable when code U0039 is present, you could experience unpredictable electronic behavior, loss of key functions, or sudden failures of safety systems. If critical modules (like ABS or airbag) are affected, driving could become unsafe. It’s best to address the issue promptly and avoid long journeys or high speeds until the fault is resolved.

Vehicles most commonly affected by code U0039 #

U0039 is a manufacturer-specific code seen across many European and some Asian vehicles, especially those using VAN or similar data bus systems. It’s more prevalent in the following brands and models:

• Peugeot (308, 407, 508, Partner, Expert)
• Citroën (C4, C5, Berlingo, Jumper)
• Renault (Clio, Megane, Kangoo, Master)
• Ford (Transit, Focus, Mondeo, C-Max)
• Opel/Vauxhall (Vivaro, Astra, Zafira)
• Some Mercedes-Benz, Toyota, and Honda models

Common mistakes & best practices for code U0039 #

A misdiagnosis can lead to wasted time and money. Avoid these common pitfalls and follow these tips:

• Don’t replace control modules prematurely; most U0039 codes are wiring or connector issues.
• Don’t ignore signs of moisture or corrosion, especially in footwells or under carpets.
• Always use OEM wiring diagrams for accurate diagnostics.
• Check for recent repairs or aftermarket installations that might have disturbed the harness.
• Clear all codes after repair and perform a full system scan to confirm communication has been restored.

Quick FAQ for code U0039 #

Q: Will disconnecting the battery clear a U0039 code?
A: No. Disconnecting the battery may erase the code temporarily, but the fault will return if the underlying issue (open circuit) isn’t fixed.

Q: Can a blown fuse cause U0039?
A: Rarely. Most communication bus circuits are fused separately from main systems, but always check related fuses just in case.

Q: Is it safe to drive with a U0039 on a long trip?
A: Not recommended. Loss of communication can disable safety-critical systems, potentially making the vehicle unsafe.