Understanding P1623 - CAN Bus High Circuit Open

When your 2021 Kia Soul sets code P1623, it indicates a problem within the Controller Area Network bus system. The CAN bus serves as the central nervous system for modern vehicle electronics, enabling communication between the engine control module, transmission control module, anti-lock braking system, and numerous other controllers. A fault in this network can affect multiple vehicle systems simultaneously.

CAN Bus Architecture Overview

The CAN bus in your Kia Soul operates as a two-wire differential network that allows multiple modules to communicate over a single shared pathway. Unlike traditional automotive wiring where each sensor connects directly to its controlling module, CAN bus architecture allows any connected device to transmit and receive messages across the network.

This standardized communication protocol dramatically reduces wiring complexity while enabling sophisticated integration between vehicle systems. When you press the accelerator, the throttle position sensor signals the PCM, which then communicates with the transmission control module to coordinate gear shifts. When you apply the brakes, the ABS module communicates wheel speed data to multiple systems that require this information.

The bus operates at different speeds depending on the message priority and system requirements. High-speed CAN handles critical powertrain communications between the engine and transmission controllers. Low-speed CAN manages body electronics including lighting, climate control, and dashboard displays.

Understanding the P1623 Fault

Code P1623 suggests that the CAN bus communication pathway has experienced a disruption that prevents normal module-to-module communication. This could stem from physical wiring damage, connector corrosion, or module internal failures that prevent proper transmission or reception of bus messages.

The specific manifestation of CAN bus failures varies significantly depending on which portion of the network is affected and the nature of the communication breakdown. Some failures cause complete network isolation where individual modules cannot communicate at all, while others result in selective communication loss affecting only certain vehicle functions.

In your 2021 Kia Soul, the most common symptoms include multiple warning lights illuminating simultaneously, gauges becoming inoperative or showing incorrect values, and various drivability complaints that defy easy diagnosis. The check engine light, ABS warning, and traction control indicator might all illuminate together as different modules detect communication failures.

Diagnostic Strategy

Professional CAN bus diagnosis requires specialized tools beyond standard OBD-II scanners. While basic code readers can retrieve the P1623 code, they cannot provide the detailed information needed to isolate the specific failure location. Automotive technicians rely on oscilloscopes and professional-grade scan tools that can monitor CAN bus message traffic in real-time.

Begin with a thorough visual inspection of the CAN bus wiring harness. The network backbone typically runs from the engine compartment through the firewall into the vehicle interior, branching to various control modules along its path. Inspect all connectors for corrosion, bent pins, or evidence of moisture intrusion. Pay particular attention to connectors located in areas prone to water exposure.

Measure CAN bus resistance at the diagnostic connector. The two CAN lines, labeled CAN-H and CAN-L, should show termination resistance of approximately 60 ohms when the battery is disconnected. This value represents the parallel combination of two 120-ohm termination resistors located at opposite ends of the network. Deviations from this specification indicate wiring faults or module failures.

Module Communication Analysis

With the vehicle connected to a professional diagnostic system, monitor CAN bus activity during operation. Healthy bus communication shows continuous message traffic with predictable timing and voltage levels. The CAN-H line should toggle between approximately 2.5 and 3.5 volts, while CAN-L oscillates between 2.5 and 1.5 volts. These voltage levels create the differential signal that receivers interpret as digital data.

When a module fails internally, it may short the CAN lines together, dramatically reducing bus resistance and causing complete communication loss across the network. Alternatively, a failed transmitter within a module might corrupt individual messages without affecting the overall bus voltage characteristics.

Isolate individual modules by disconnecting them one at a time while monitoring bus communication. If bus function returns when a particular module is disconnected, that module is likely the source of the problem. However, some failures are intermittent and only manifest under specific temperature or vibration conditions.

Network Repair Considerations

Once the faulty module or wiring section is identified, repair options depend on the specific failure mode. Wiring repairs involve splicing and sealing connections using manufacturer-approved methods. Moisture-damaged connectors often require replacement of the entire harness section, which can be expensive given the complexity of modern vehicle wiring.

Module replacement requires professional programming to integrate the new component into the existing CAN network. The replacement module must learn the vehicle's specific configuration data, including VIN, calibration information, and network node identification. Without proper programming, replacement modules cannot participate in CAN bus communication.

For vehicles with extensive water damage or those showing multiple module failures, total harness replacement might represent the most practical solution, though cost considerations often lead owners toward vehicle replacement rather than extensive repair.