OBD-II trouble code
U0012: Medium Speed CAN Communication Bus (+) Open
A break in the CAN-High conductor of the medium-speed network. Which modules can still be reached tells you where the break is, and a single resistance reading at the diagnostic connector confirms it before any trim comes off.
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?
A CAN network moves data as a difference in voltage between two wires rather than as a voltage on one. Both sit at roughly 2.5 volts when nothing is being transmitted. To send a dominant bit, the transmitting module pushes one wire up and pulls the other down at the same instant, and every listening module reads only the gap between them. U0012 reports that the wire that gets pushed up — CAN-High, the positive conductor — is broken somewhere on the medium-speed bus.
The most useful property of an open on a shared bus is that it does not silence the network evenly. It cuts the bus into two electrically separate lengths of wire, and the modules on each side can still hear each other perfectly. What they cannot do is hear across the gap. That turns the list of modules a scan tool can still reach into a map: overlay it on the bus topology diagram for the vehicle and the break sits between the group you can talk to and the group you cannot. This is faster and far less destructive than opening the harness, and it is a step worth completing before any panel is removed.
A single meter reading confirms the picture. With everything asleep, resistance measured across the two bus wires at the diagnostic connector should read close to 60 ohms, because the network is terminated at each end by a 120 ohm resistor and the meter sees the pair in parallel. An open in the pair takes one of those terminators out of the measurement, so the reading roughly doubles. Around 120 ohms where 60 was expected is strong confirmation of an open before a single connector is disturbed.
One thing this code does not necessarily mean is that the affected features are dead. Many body and comfort networks use a fault-tolerant CAN implementation designed specifically to survive one wire failing: the transceivers detect the loss and fall back to running single-ended on the surviving conductor referenced to ground. Performance drops, noise immunity is gone, and the system logs the fault — but the car often behaves close to normally. That explains the common complaint of a warning light with almost nothing else wrong, and it is also the reason not to postpone the repair. The redundancy that is masking this failure has already been spent, and the next fault on the remaining wire takes the whole segment down at once.
Common causes
- CAN-High conductor cut, chafed through, or pulled apart at a splice
- Backed-out or corroded CAN-High terminal in a bus connector
- Connector left unseated after bodywork, audio installation or accessory fitting
- Rodent damage to the harness where it crosses a bulkhead or wheel arch
- Water tracking into a door or tailgate connector and opening the circuit intermittently
- Module failing internally in a way that opens its section of the bus
- Harness damage from collision repair that pinched or severed the pair
Symptoms
- A block of features going dead together while unrelated ones work normally
- Scan tool able to reach some modules and not others on the same network
- Airbag, ABS or other warning lamps illuminated because a module lost its status message
- Warning light with surprisingly little functional loss on fault-tolerant networks
- Multiple lost-communication U-codes stored across several modules
- Engine typically starts and runs normally
Diagnostic steps
- 1.Scan every module and write down which ones respond and which do not. Do this before touching anything, because the pattern is the most valuable evidence the fault produces.
- 2.Obtain the bus topology diagram for the specific vehicle and mark the reachable and unreachable modules on it. The break lies between the two groups.
- 3.With the ignition off and the network asleep, measure resistance across the two bus wires at the diagnostic connector. Roughly 60 ohms is normal; roughly 120 ohms indicates the pair is open.
- 4.Repeat that resistance measurement at an accessible connector on the far side of the suspected break to confirm which length of wire has lost its terminator.
- 5.Check whether the vehicle uses fault-tolerant CAN on this network. If it does, expect the system to be partially working and do not treat that as evidence against an open.
- 6.Inspect the connectors bracketing the suspected section for a backed-out terminal, green corrosion or a partially seated lock, working from the most exposed location inward.
- 7.After repair, confirm every module reappears on the scan tool and that the resistance reading has returned to about 60 ohms before clearing codes.
Repair cost
$100 – $700
Locating the break is where the money goes on network faults, typically $100 to $250 of diagnostic time. Repairing a terminal or splicing a wire once found is often $120 to $400. If a module is internally opening the bus, replacement and programming runs $400 to $700 and higher on European platforms.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
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.