All Network Communication DTC Codes — Complete OBD-II Diagnostic Guide (2026)
This comprehensive reference covers every Network Communication diagnostic trouble code (DTC) — 9 codes total. Each code includes what triggers it, common symptoms, step-by-step diagnostic procedures, and repair guidance.
Detailed DTC Code Guides
U0073: The engine control module (ECM) cannot talk to other control modules on Bus A. This shuts down data exchange, so the vehicle may run in limp mode or display multiple other codes.
U0073 means the primary communication bus on your vehicle is offline. Learn quick checks, root causes, cost estimates, emissions impact, and DIY repair...
- Inspect connectors and fuses
- Perform a 60‑second scan
- Check ground and power supplies
- ...see all 6 steps →
U0100: The U0100 code tells you that the engine control module (ECM) or power‑train control module (PCM) isn’t receiving data from another control unit on the vehicle’s communication bus. This usually points to a wiring problem, a bad connector, or a failed module.
Discover what the U0100 code means, why it shows up, and how to fix it yourself. Includes step‑by‑step diagnostics, cost estimates, emissions impact, and...
- Visual Inspection of Connectors
- Check Battery Voltage and Ground
- Scan Live Data for Communication Errors
- ...see all 5 steps →
U0101: The power‑train control module (TCM) cannot receive data from the engine control module (ECM) over the CAN bus. This loss of communication stops the TCM from commanding the transmission, leading to limp‑mode or erratic gear changes.
U0101 means the transmission control module can’t talk to the engine computer. Learn step‑by‑step diagnosis, common causes, repair costs, emissions impact,...
- Visual inspection of obvious faults
- Battery voltage and chassis ground check
- Live CAN‑bus activity scan
- ...see all 6 steps →
U0121: U0121 signals that the Engine Control Module (ECM) cannot exchange data with the Anti‑Lock Braking System (ABS) control module. The loss of communication may stem from a damaged ABS module, a broken wiring harness, a poor connector, or outdated software.
A step‑by‑step DIY guide for the U0121 trouble code. Learn how to pinpoint lost communication with the ABS control module, understand safety risks, estimate...
- Step 1 – Visual Inspection
- Step 2 – Power & Ground Verification
- Step 3 – CAN‑Bus Continuity Test
- ...see all 5 steps →
U0126: The PCM (power‑train control module) can’t receive data from the steering‑angle sensor module. This loss of communication prevents the vehicle from using accurate wheel‑angle information for stability‑control and steering‑assist functions.
U0126 code means the steering angle sensor isn’t talking to the PCM. Get a quick diagnosis, step‑by‑step repair, cost breakdown, emissions impact, and see...
- Inspect connectors for looseness or corrosion
- Examine wiring harness for chafing or shorts
- Read live data for steering‑angle sensor output
- ...see all 6 steps →
U0128: The power‑train control module (PCM) cannot receive data from the park brake control module. This usually points to a wiring fault, a bad connector, or a failed module that interrupts the CAN‑bus communication.
U0128 means the park brake control module isn’t talking to the PCM. Learn quick checks, root causes, repair costs, and DIY steps for Toyota, Honda, Ford, Chevy.
- Check for obvious issues
- Scan live data and freeze frames
- Verify power and ground at the module
- ...see all 6 steps →
U0140: The U0140 diagnostic trouble code signals that the Powertrain Control Module (PCM) has lost communication with the Body Control Module (BCM). Without this link, the BCM cannot manage lighting, door locks, and other comfort systems, and the PCM may limit engine output as a precaution.
U0140 code means the PCM can’t talk to the Body Control Module. Get a quick diagnosis, see which Toyota, Honda, Ford and Chevy models are affected, and...
- Check for Loose or Corroded BCM Connector
- Verify Battery Voltage and Ground Integrity
- Scan Live Data for PCM‑BCM Heartbeat
- ...see all 5 steps →
U0155: The power‑train control module (PCM) cannot receive data from the instrument panel cluster (IPC). The loss of communication disables the speedometer, tachometer, warning lights, and may trigger a MIL.
The U0155 DTC means the instrument panel cluster (IPC) isn’t talking to the PCM. Learn step‑by‑step diagnostics, vehicle‑specific tips, safety warnings,...
- Inspect the IPC harness
- Check battery voltage and ground
- Read live IPC status
- ...see all 6 steps →
U1000: The power‑train control module (PCM) cannot receive or send data on the vehicle’s CAN (Controller Area Network) bus. This stops modules like the engine, transmission, ABS and traction‑control from sharing sensor information, which can cause erratic operation or limp‑mode.
The U1000 DTC means the CAN bus isn’t talking. Learn why it happens on Toyota, Honda, Ford, Chevrolet, get a 60‑second quick check, step‑by‑step repair,...
- Verify power supply and grounding
- Inspect CAN connectors and harnesses
- Read all stored codes
- ...see all 7 steps →
Understanding Network Communication Codes
Network Communication diagnostic trouble codes are set by your vehicle's onboard computer (ECU/PCM) when it detects a problem in the communication system. Here's what you need to know:
- How codes are triggered: Sensors monitor communication performance. When readings fall outside expected ranges, the ECU stores a DTC and illuminates the check engine light.
- Freeze frame data: When a code is set, the ECU also records engine conditions at that moment. This data helps pinpoint the exact cause.
- Pending vs. confirmed: A code becomes "confirmed" after multiple drive cycles with the same fault. Pending codes may clear on their own.
Tools You'll Need for Diagnosis
- OBD-II scanner — Essential for reading and clearing codes. A basic scanner works for most codes.
- Digital multimeter — For testing sensor voltages, resistance, and continuity.
- Repair manual — Vehicle-specific wiring diagrams and specifications.
- Safety equipment — Gloves, safety glasses, jack stands for under-vehicle work.
When to See a Mechanic
While many communication codes can be diagnosed at home, see a professional if:
- The code is critical severity — driving may cause further damage
- Multiple related codes are present — suggests a deeper systemic issue
- You don't have the tools or experience for the required diagnostic steps
- The repair involves fuel system, exhaust, or catalytic converter work