Step 1 : Pls. check below logs and trc files

 /u01/app/oracle/product/10.2.0/db_1/test/sysman/log/
emagentfetchlet.log  emoms.log            emoms.trc.5
emagentfetchlet.trc  emoms.log.1          emoms.trc.6
emagent.log          emoms.trc            emoms.trc.7
emagent.trc          emoms.trc.1          emoms.trc.8
emdb.nohup           emoms.trc.10         emoms.trc.9
emdb.nohup.lr        emoms.trc.2          nmctestdb1522/
emdctl.log           emoms.trc.3
emdctl.trc           emoms.trc.4

Step 2 : If below file has error

emoms.trc

2006-09-26 11:52:13,097 [HttpRequestHandler-21362147] ERROR
eml.OMSHandshake getParameterFromDB.402 - Io exception: The Network
Adapter could not establish the connection
2006-09-26 11:52:13,097 [HttpRequestHandler-21362147] ERROR
eml.OMSHandshake processFailure.619 - OMSHandshake failed.(AGENT URL =
http://testdb:3938/emd/main)(ERROR = INTERNAL_ERROR)

Step 3 : Open below file and check parameters . put correct parameter (if you change host file,machine ip etc)

vi /u01/app/oracle/product/10.2.0/db_1/test/sysman/config/emoms.properties

oracle.sysman.emSDK.svlt.ConsoleServerName=testdb_Management_Service
oracle.sysman.eml.mntr.emdRepPwd=92c4690e0fc4fe86
emdrep.ping.pingCommand=/usr/sbin/ping6 <hostname>
oracle.sysman.eml.mntr.emdRepPort=1521
oracle.sysman.eml.mntr.emdRepDBName=testdb
oracle.sysman.eml.mntr.emdRepPwdSeed=7067240962669395015
oracle.sysman.emSDK.svlt.ConsoleMode=standalone
oracle.sysman.emRep.dbConn.statementCacheSize=30
oracle.sysman.db.isqlplusUrl=http\://testdb\:5560/isqlplus/dynamic
oracle.sysman.emSDK.svlt.ConsoleServerPort=1158
oracle.sysman.eml.mntr.emdRepRAC=FALSE
oracle.sysman.emSDK.emd.rt.useMonitoringCred=true
oracle.sysman.eml.mntr.emdRepPwdEncrypted=TRUE
oracle.sysman.db.isqlplusWebDBAUrl=http\://testdb\:5560/isqlplus/dba/dynamic
oracle.sysman.emSDK.svlt.ConsoleServerHost=testdb
oracle.sysman.eml.mntr.emdRepDBID=3664119539
oracle.sysman.emSDK.svlt.ConsoleServerHTTPSPort=1158
oracle.sysman.eml.mntr.emdRepServer=testdb
oracle.sysman.eml.mntr.emdRepSID=testdb

oracle.sysman.eml.mntr.emdRepConnectDescriptor= (DESCRIPTION\=(ADDRESS_LIST\=(ADDRESS\=(PROTOCOL\=TCP) (HOST\=testdb)(PORT\=1522)))(CONNECT_DATA\= (SERVICE_NAME\= testdb)))