Troubleshooting AppDynamics agent connection issues begins with verifying that the agent can reach the Controller by checking network connectivity, DNS resolution, and required ports. Ensure the Controller host, port, and account access key are correctly configured in the agent settings, as incorrect values commonly cause authentication failures. Reviewing agent logs helps identify issues such as SSL handshake errors, certificate mismatches, or incompatible Controller and agent versions. Firewall or proxy restrictions should also be checked, especially in secured enterprise environments where outbound traffic may be blocked. Validate that application, tier, and node names match what is defined in the Controller and that sufficient licenses are available. After correcting any configuration or network issues, restarting the agent or application typically re-establishes a stable connection.