Discipline 4, "Determining Root Cause," of the 8D Troubleshooting Methodology involves in-depth analysis, hypothesis testing, and verification to find the fundamental origin of a problem. Within the Avaya tiered support model, this level of complex investigation is the designated responsibility of Avaya Tier 3 support. Tier 3 consists of senior engineers and subject matter experts who are engaged when an issue cannot be resolved by lower tiers and requires deep product knowledge to identify the underlying cause and develop a permanent solution.

A. Business Partners: Business Partners typically provide Tier 1 or Tier 2 level support and escalate complex, unresolved issues to Avaya, rather than performing the final root cause analysis.

B. Avaya Tier 2: Tier 2 support focuses on resolving known issues and performing initial diagnostics. They escalate to Tier 3 when the problem's root cause is unknown or requires engineering-level expertise.

C. Third Party Support: This group is responsible for non-Avaya components within a solution. While they may assist in the investigation, Avaya Tier 3 retains ownership for determining the root cause of an Avaya product issue.

1. Avaya Learning, Course 72300C: Troubleshooting Methodology for Avaya Aura® Communication Applications. This course, which prepares for the 72301X exam, explicitly outlines the roles of different support tiers within the 8D framework. Module 2, "The 8D Troubleshooting Process," details that Tier 3 is responsible for Disciplines 4 (Determine Root Cause) and 5 (Choose and Verify Permanent Corrective Actions).

2. Avaya Aura® Communication Applications Support Exam (72301X) Study Guide, Version 4.0. The section on "Avaya Support Process and Procedures" describes the escalation path. It defines Tier 3 as the final point of internal escalation for complex problem resolution, which inherently includes root cause analysis for new or systemic faults.