What's the difference between U0012 and U0003?

Both are 'CAN-High open' faults, but on different networks. U0003 is the open on the high-speed bus that carries the engine, transmission, and ABS modules; U0012 is the same kind of open on the slower medium-speed bus that carries body, comfort, and infotainment modules. That's why U0012 usually affects convenience features rather than disabling the car.

Can I drive with U0012?

Often yes, because the medium-speed bus typically handles comfort and convenience systems rather than core driving controls. The engine and brakes usually still work, but you may lose climate control, infotainment, or accessory functions, and some displays may not update. Have it diagnosed soon, since the same wiring fault can spread.

Why did several features stop working at once?

An open on the CAN-High wire silences every module downstream of the break at the same time. Since those modules share one bus, all the features they control can go dead together even though each part is fine — the common link between them is what failed.

Is U0012 usually a wiring problem or a bad module?

Most often it's a wiring or connector fault — a broken CAN-High wire, a corroded terminal, or a loose connector. A failed module can also open the bus segment, but technicians confirm that only after testing the wiring, because replacing a module that wasn't the cause is an expensive mistake.

OBD-II trouble code

U0012: Medium Speed CAN Communication Bus (+) Open

The positive (CAN-High) wire of the medium-speed communication bus has an open — a break, loose pin, or disconnected segment. Modules on that bus can no longer exchange messages reliably.

Quick facts

System: Network
Category: Network Communication
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $100 – $700
DIY difficulty: Shop recommended

What does U0012 mean?

U0012 indicates an open circuit on the positive line — CAN-High or CAN(+) — of the medium-speed CAN bus. Many vehicles run more than one communication network: a high-speed bus that links the critical powertrain, ABS, and stability modules, and a slower medium-speed bus that handles body, comfort, infotainment, and convenience modules where split-second timing matters less. U0012 is set when the CAN(+) wire on that medium-speed network is broken or disconnected, so the signal can no longer pass between the modules that share it.

A CAN bus carries data as the voltage difference between its two wires, CAN-High and CAN-Low. When CAN-High goes open, that differential collapses or becomes intermittent and the modules downstream of the break stop hearing each other. Unlike a generic 'bus performance' code, U0012 points specifically at an open on the positive conductor — typically a cut or chafed wire, a corroded or backed-out terminal, a separated splice, or a connector that has worked loose. It is the medium-speed counterpart to U0003 on the high-speed bus.

Because the medium-speed bus usually carries comfort and convenience functions rather than core driving systems, U0012 is generally less severe than a high-speed bus fault. The car will usually still start and drive, but you may lose features such as climate control, infotainment, power accessories, lighting functions, or driver displays, and several lost-communication U-codes may set together. It is diagnosed electrically — locating the open in the CAN(+) wiring — not by swapping modules on a guess.

Common causes

Broken or cut CAN-High wire on the medium-speed bus
Corroded, loose, or backed-out terminal in a bus connector
Separated or failed splice in the CAN(+) wiring
Connector unplugged or not fully seated after prior service
Water intrusion and corrosion opening the circuit intermittently
Harness damage from rodents, road debris, or accident repair
Internally failed module dropping the bus segment open

Symptoms

One or more comfort/convenience features stop working (climate, infotainment, accessories)
Driver information displays freeze, blank out, or show fault messages
Multiple lost-communication U-codes stored alongside U0012
Scan tool cannot reach modules on the medium-speed bus
Intermittent feature loss that tracks with vibration or moisture
Engine usually still starts and runs normally

Diagnostic steps

1.Scan every module and record all stored codes; the pattern of lost-communication codes helps show which bus segment is open.
2.Identify which modules sit on the medium-speed bus for the specific vehicle using wiring diagrams.
3.Key on, measure CAN-High voltage along the bus; it should rest near 2.5 volts. A reading that goes open or floats past a certain point helps localize the break.
4.With key off and battery disconnected, check continuity of the CAN-High wire between modules to find the open.
5.Inspect connectors and splices for corrosion, backed-out terminals, and water intrusion, especially after recent service.
6.Repair the open and clear the codes, then confirm the affected modules reappear on the scan tool and U0012 does not return.

Repair cost

$100 – $700

Diagnosis to locate the open commonly runs $100-$250. A wiring or connector repair to restore CAN-High continuity is often $150-$500. If a failed module is breaking the bus, replacement with programming can run $400-$700+, higher on luxury and European platforms.

Estimate your repair

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Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.

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DIY vs shop

Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.

Related codes

U0010 U0011 U0003 U0019 U0140 U0155