Smog Repair Technician Practice Exam 2025 - Free Smog Technician Practice Questions and Study Guide

Pin 4 on the OBD-II connector is designated for chassis ground. This pin provides a common ground reference for the diagnostic equipment used to interface with the vehicle's onboard computer systems. Establishing a solid ground connection is crucial for accurate data transmission and communication between the vehicle's control modules and the diagnostic tools. By connecting to chassis ground, technicians can ensure that the readings they receive are reliable and not influenced by electrical noise or floating ground issues.

In the context of the OBD-II connector, other pins have specific functions related to communication protocols or vehicle power supply. Pin 5 is typically used for signal ground (which is different from chassis ground), while pin 12 is generally not utilized for ground purposes. Pin 16 is primarily used for power supply (usually 12V) to the diagnostic tools. Understanding the roles of these pins is vital for effective vehicle diagnostics and repairs.