U0027
NetworkLow Speed CAN Communication Bus (-) shorted to Bus (+)
The ECU monitors the two wires that make up the low-speed CAN data bus, CAN-High and CAN-Low. These wires normally carry mirror-image voltages and sit a little apart from each other. When the module sees those two lines effectively touching, with their voltages collapsing together, it logs U0027 and flags a short between the positive and negative bus lines. For you, that means the body modules that chat over this network (windows, locks, lights, dash) can't talk properly, so bits of the car start behaving oddly or stop working altogether.
ⓘ Information only. This page provides general educational information about fault code U0027. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does U0027 mean?
U0027 is a Network (CAN bus, module communications) fault code. It indicates: Low Speed CAN Communication Bus (-) shorted to Bus (+).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Warning light on the dash, sometimes a generic CAN or network error message depending on the make
- • Windows, sunroof, central locking or boot release working when they feel like it, or not at all
- • Interior and exterior lights flickering, staying dark, or refusing to respond to the switch
- • Instrument cluster gauges freezing, sweeping to zero, or jumping around
- • Key fob lock and unlock commands ignored
- • Functions coming back briefly after a restart then dropping out again once the car's been running
Possible causes
Causes commonly associated with U0027, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Chafed or rodent-chewed wiring where the CAN-H and CAN-L conductors have rubbed through their insulation and touched. This is the usual culprit, often where the loom passes a sharp edge or a door grommet
- 2. Corroded or pushed-back connector pins bridging the two lines together, common at door connectors and tailgate hinges that flex constantly
- 3. Water getting into a connector or module, salty winter roads make this far worse and a damp plug can short the two lines internally
- 4. A failed body control module or similar unit with an internally shorted CAN transceiver, the wiring is fine but the chip inside has let go
- 5. Poor chassis earth dragging the bus reference around so the lines pull together under load
- 6. Aftermarket alarm, dashcam or stereo install spliced badly into the low-speed CAN, a sloppy tap can short the pair
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull the codes and note anything else stored. U0027 rarely turns up alone. Companion codes like U0100, U0140 or U0155 tell you which modules have gone quiet and point you toward the section of bus that's affected
- 2. Get the harness up on a lift and follow it by eye and by hand. Look at door grommets, the boot loom, anywhere the wire flexes or rubs. Rodent damage and melted insulation are obvious once you spot them
- 3. With the battery disconnected, measure resistance between CAN-H and CAN-L. A healthy bus reads around 60 ohms thanks to the two terminating resistors. A reading down near zero or below 5 ohms confirms the lines are shorted together
- 4. Start unplugging modules one at a time, BCM, door modules, instrument cluster, and re-measure after each. When the resistance jumps back to normal, the last thing you disconnected owns the short
- 5. Power the system up and check CAN-H and CAN-L voltages. They should sit either side of roughly 2.5V at idle. If both have collapsed onto each other, you're looking at a live short rather than an intermittent one
Common questions about U0027
Can I sort this myself in the driveway? +
If you find obvious chafed or chewed wiring, repairing it properly with soldered joins and heatshrink is within reach of a confident DIYer who owns a multimeter. The hard part is finding the short in the first place. It can hide inside a door connector or up under the dash, and chasing it without a wiring diagram is miserable. If your resistance test points at a module rather than the loom, you're into replacing and coding a unit, which needs a garage with the right software. Realistic shout: hunt for damaged wiring first, and if the harness is clean, hand it to a specialist.
If I clear the code does it stay gone? +
No. A short is a physical fault. Wipe the code and it'll be back the moment the two lines touch again, which on a hard short is instantly and on an intermittent one might be the next time you slam a door or hit a bump. Clearing codes is only useful as a final check after you've actually fixed the wiring or replaced the dud module, to confirm everything comes back to life.
What happens if I just leave it? +
Depends what's dropped off the network. If it's only your window switches sulking, it's annoying rather than dangerous. The problem is that a CAN short can take out lighting, locks or warning lamps, and a car you can't lock or one with dead headlights at night is a real safety issue. There's also a knock-on risk: a short can drag the whole bus down and confuse other modules, so a small fault sometimes snowballs into half the car misbehaving. It won't fix itself, and a chafed wire usually gets worse as it keeps rubbing.
How quickly do I need to get this looked at? +
Treat it as soon-ish rather than panic, with one caveat. If lights, indicators or the brake-related warnings are affected, get it seen before driving again, particularly in the dark. If it's just convenience stuff playing up and everything safety-critical works, you can book it in within a few days, but don't let it drag on for weeks because moisture and flexing wires tend to turn a niggle into a dead bus.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →