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U0027 — Low Speed Can Communication Bus Shorted Bus

Generic OBD-II · all makesNetwork / communication

U0027 indicates that a low-speed controller area network (CAN) bus has been detected as shorted—CAN-High to CAN-Low, to ground, or to battery voltage. This network typically links body and convenience modules such as door, seat, climate, and mirror modules. The short prevents normal module-to-module communication and can disable multiple accessory systems.

Symptoms

What causes it — most likely first

1. Wiring harness short

The low-speed CAN pair is shorted together or to ground/battery due to chafing, pinching, connector damage, or water intrusion into a harness. This is the most common root cause of U0027.

2. Corroded connector or splice pack

Moisture or terminal fretting inside a module connector or inline splice can bridge CAN-High and CAN-Low or either line to power/ground, producing a bus-short code.

3. Failed body/convenience module

A door module, seat module, climate control module, or similar low-speed CAN node can fail internally and short the bus lines, pulling the entire network down.

4. Aftermarket electrical device

Non-factory accessories tapped into low-speed CAN wires can load or short the bus, causing communication faults across the network.

5. Control module internal bus driver failure

A PCM, BCM, or gateway module with an internal low-speed CAN transceiver fault is possible, but it is the least likely cause and should only be considered after wiring and other modules are ruled out.

How it's diagnosed

  1. Confirm the code and check which modules communicate; note any modules that are offline or report no response.
  2. Visually inspect accessible low-speed CAN wiring, connectors, and splice packs for chafing, corrosion, bent pins, or evidence of water entry.
  3. Disconnect low-speed CAN modules one at a time while monitoring bus voltage and resistance; if the short clears when a specific module is unplugged, that module is the likely source.
  4. Measure CAN-High and CAN-Low voltage at a convenient access point; readings near 0 V or battery voltage indicate a short to ground or power, respectively.
  5. If the short remains with all modules disconnected, perform pinpoint continuity and isolation tests on the harness to locate and repair the wiring fault.

Code facts & related codes

SystemNetwork / communication
StandardizationSAE generic — identical on every make
Related codesU0020 — Low Speed Can Communication Bus PerforU0021 — Low Speed Can Communication Bus OpenU0022 — Low Speed Can Communication Bus LowU0023 — Low Speed Can Communication Bus HighU0024 — Low Speed Can Communication Bus OpenU0025 — Low Speed Can Communication Bus Low

Frequently asked

Can I keep driving with U0027?

Basic engine operation is usually unaffected, but body and convenience functions may be inoperative or behave erratically. The fault should be diagnosed promptly to avoid further electrical issues.

Do I need a new PCM for U0027?

Usually no. A failed PCM is one of the least likely causes of this code. U0027 describes a shorted bus, and the problem is most often found in the wiring, a connector, or a body/convenience module on the low-speed network.

Why do some shops recommend replacing the PCM for this code?

Communication DTCs are sometimes misread as a computer failure. Because U0027 specifically means the low-speed CAN bus itself is shorted, replacing the PCM rarely fixes it unless the PCM has already been proven to be the source of the short.

Can an aftermarket alarm or stereo cause U0027?

Yes. If an aftermarket device has been connected to low-speed CAN wires, it can load down or short the bus and trigger this code. Removing or properly isolating the device often resolves the fault.