Guide

What is a U code? Network communication codes

A code that starts with U is a network communication code. It usually means one control module stopped hearing messages it expected from another module on the car's data network.

Key facts

Key fact	Detail
First letter U	Network communication (P is powertrain, B body, C chassis)
U0xxx	Generic codes defined by SAE J2012
U01xx	Lost communication with a named module, for example U0100 (ECM/PCM)
Network	Most modern cars use CAN, available at the OBD2 port on pins 6 and 14
Check first	Battery voltage, fuses, power and ground, then network wiring

U codes describe a communication problem, not a failed part. Diagnose before replacing any module.

What the U stands for

The first character of an OBD2 trouble code shows the system: P for powertrain, B for body, C for chassis and U for network communication. A 0 as the second character means a generic code whose title is standardized by SAE J2012. A 1 usually means a manufacturer specific code.

Lost communication codes

The most searched U codes are lost communication codes. The module that stores the code is reporting that another module went silent, so the module named in the code is the one to investigate, not the one that stored it.

Code	Title
U0100	Lost Communication With ECM/PCM A
U0101	Lost Communication With TCM
U0121	Lost Communication With Anti-Lock Brake System (ABS) Control Module
U0140	Lost Communication With Body Control Module

See also: U0100 guide · U0101 guide · U0121 guide · U0140 guide

How the CAN network works

Modules share a two wire bus called CAN (Controller Area Network). The two wires, CAN high and CAN low, are also brought out to the OBD2 port on pins 6 and 14, which is how a scanner talks to the car.

High speed CAN is terminated at each end with a 120 ohm resistor. A manufacturer bulletin gives the matching workshop check: with the battery disconnected, resistance across the network should be close to 60 ohms at a module and close to 120 ohms at a terminating module. Readings far from these values point to an open circuit, a short or a missing terminator.

Common causes

Low battery voltage. Manufacturer bulletins published by NHTSA note that low voltage can set several U codes at once.
A blown fuse, or a loose or corroded power or ground connection at a module.
Open or short circuits in the CAN wiring, or bent or pushed out connector pins.
A faulty module, which is possible but should be confirmed only after power, ground and wiring checks.

What to check first

Write down every code and its freeze frame before clearing anything.
Check the battery and charging voltage (see the battery voltage guide below).
Check the fuses and grounds for the module named in the code.
Clear the codes and see which ones return. Codes stored during a low voltage event may not return once the battery is charged and healthy.
If the codes return, have the network wiring and module checked with service information for your car.

If a module has lost communication, a scanner may also be unable to talk to it. Many U codes are stored in body, brake or other modules that generic OBD2 does not read, so what you can see depends on your car and its profile in the app.

Try it on your own car

OBD Master Pro turns your Android phone and a generic ELM327 adapter into a code reader: read and clear codes, view freeze frame and live data, and check emissions readiness.

Keep reading

Common codes: P0171 System Too Lean · P0420 Catalyst Efficiency · P0300 Random Misfire · P0455 EVAP Large Leak

All OBD2 guides · OBD2 by car brand · OBD2 trouble code guides

Sources

Published October 24, 2026. Updated October 24, 2026. General information only. Details generally vary by model, year and market, so always follow the owner's manual and service information for your vehicle.