环境：
2台SQL Server 2012 Always ON服务
现象：
Always ON服务正常，同步正常，应用正常。
Second Server Server Agent 启动数秒后就自动停止。
处理过程：
1、查看Windows 系统日志，显示如下信息：
2、在SQL Studio 中查看当前Error Log
Date,Source,Severity,Message
09/25/2018 13:54:07,,Information,[098] SQLServerAgent terminated (normally)
09/25/2018 13:54:07,,Error,[382] Logon to server 'domain\SQLInstance' failed (DisableAgentXPs)
09/25/2018 13:54:07,,Error,[165] ODBC Error: 0<c/> Data source name not found and no default driver specified [SQLSTATE IM002]
09/25/2018 13:54:07,,Error,[165] ODBC Error: 0<c/> Data source name not found and no default driver specified [SQLSTATE IM002]
09/25/2018 13:54:07,,Error,[165] ODBC Error: 0<c/> Data source name not found and no default driver specified [SQLSTATE IM002]
09/25/2018 13:54:07,,Error,[165] ODBC Error: 0<c/> Data source name not found and no default driver specified [SQLSTATE IM002]
09/25/2018 13:54:07,,Error,[165] ODBC Error: 0<c/> Data source name not found and no default driver specified [SQLSTATE IM002]
09/25/2018 13:54:07,,Error,[000] Unable to connect to server 'domain\SQLInstance'; SQLServerAgent cannot start
09/25/2018 13:54:07,,Error,[150] SQL Server does not accept the connection (error: 0). Waiting for Sql Server to allow connections. Operation attempted was: Verify Connection On Start.
09/25/2018 13:53:37,,Error,[150] SQL Server does not accept the connection (error: 0). Waiting for Sql Server to allow connections. Operation attempted was: Verify Connection On Start.
09/25/2018 13:53:07,,Error,[150] SQL Server does not accept the connection (error: 0). Waiting for Sql Server to allow connections. Operation attempted was: Verify Connection On Start.
09/25/2018 13:52:37,,Error,[150] SQL Server does not accept the connection (error: 0). Waiting for Sql Server to allow connections. Operation attempted was: Verify Connection On Start.
09/25/2018 13:52:07,,Error,[150] SQL Server does not accept the connection (error: 0). Waiting for Sql Server to allow connections. Operation attempted was: Verify Connection On Start.
09/25/2018 13:52:07,,Information,[495] The SQL Server Agent startup service account is domain\dbadmin.
09/25/2018 13:52:07,,Information,[100] Microsoft SQLServerAgent version 11.0.3000.0 (X64 unicode retail build) : Process ID 1008
3、结合以上错误，似乎都指向ODBC，打开ODBC，发现ODBC 64bit 中Driver为空
正常是应该有SQL Server和 SQL Native Client 11这两个驱动的。
4、在SQL Server 2012 安装包中找到sqlncli.msi 重新安装SQL Native Client 11(注意选择重装，而不是修复)。
5、再次在ODBC中查看，SQL Native Client 11已经有了。
6、重启SQL Agent ，观察几分钟后没有再出现自动停止的现象，问题解决。
后面有看到一篇介绍ODBC驱动丢失的文章：http://www.cnblogs.com/chinalantian/archive/2016/06/13/5581644.html
了解到可能是卸载某些软件的时候会把ODBC中的部分驱动也删除，咨询客户表示近期有删除一些软件。
特此记录！