U0031 — Vehicle Communication Bus Low

Generic OBD-II · all makesNetwork / communication

U0031 is a network communication code indicating the vehicle's primary communication bus (typically the CAN bus) has a low voltage condition, meaning the signal voltage on the data network is below specification. This affects communication between the PCM/ECM and other control modules, preventing proper data exchange.

Symptoms

Multiple warning lights (check engine, ABS, airbag) illuminated simultaneously
Intermittent or complete loss of communication with various control modules
Vehicle may crank but not start, or run poorly with stuttering
Some vehicle functions (radio, climate control, gauges) may be inoperative or erratic
Stored multiple U-codes indicating which modules cannot communicate
In some cases, the vehicle operates normally but the check engine light remains on

What causes it — most likely first

1. Failed or failing control module on the network

A defective ECU, TCM, BCM, or other module can pull the bus voltage low by internally shorting to ground or failing to properly communicate.

2. Damaged or corroded wiring harness

Chafed, rubbed, or corroded wires in the communication bus circuit can create shorts to ground or increase resistance beyond spec.

3. Short to ground on the communication bus wires

One or more of the CAN bus wires (CAN High or CAN Low) is shorted to ground, pulling the voltage down and preventing proper communication.

4. Poor electrical connection at module connector

Corroded, loose, or damaged pins at a module connector can cause intermittent or low voltage conditions on the network.

5. Failed bus termination resistor

Modern CAN networks use 120-ohm resistors at each end; if one fails open or changes value, it can cause communication issues.

6. PCM/ECM failure (rare)

The powertrain control module itself can fail, though this is one of the least common causes and should be diagnosed last after all wiring and other modules are ruled out.

How it's diagnosed

Connect an OBD-II scanner and retrieve all stored U-codes; note which modules are present versus which are reporting as 'no communication'
Perform a visual inspection of the main data bus wiring harness, looking for damage, chafing, corrosion, or evidence of prior repair
Check the voltage on the CAN High and CAN Low wires at the DLC (Diagnostic Link Connector) with a multimeter; CAN Low should read around 1.5-2.5V with key on, CAN High around 2.5-3.5V
Check for shorts to ground by testing each bus wire to ground with a multimeter in resistance mode; a low ohm reading indicates a short
Isolate modules one at a time by disconnecting them to see if bus voltage returns to normal, identifying the module causing the short
If no wiring or module issue is found, check power and ground circuits to each affected module with a multimeter to ensure proper voltage supply

Code facts & related codes

SystemNetwork / communication

StandardizationSAE generic — identical on every make

Vehicles this code applies to

The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:

2013 Scion FR-S 2014 Scion FR-S 2017 Toyota 86 2018 Toyota 86

Frequently asked

Is it safe to drive with a U0031 code?

It depends on the severity and which systems are affected. Some vehicles may drive normally while others may experience drivability issues, loss of critical functions (brakes, airbags), or no-start conditions. Have it diagnosed promptly.

Will a parts store replace my PCM/ECM for this code?

Some may suggest it, but replacing the PCM/ECM for U0031 is rarely the correct fix. The PCM is the last item to suspect after all wiring, connectors, and other modules have been thoroughly checked.

Do I need a special scan tool to diagnose this?

A basic OBD-II scanner can retrieve the code and identify which modules are not communicating. However, a professional-grade scan tool with network diagnostics (like a factory-level scanner) is often needed to perform detailed bus voltage measurements and module-specific tests.

Can a weak car battery cause this code?

Extremely low battery voltage (below 9V) can sometimes cause communication issues, but this is uncommon. More often, low battery voltage will cause multiple random codes rather than a specific bus low condition. Ensure the battery and charging system are healthy before focusing on the network.