The general error codes are output in the HULFT Receive, Send, and Request Acknowledge program (STC) as [end code]-[details-code]. From here you can get an approximation of what caused the error. You can check via "Error Codes - Messages Manual" for all versions of Mainframe.

Also, for the other enabled codes, all platforms feature TCIP and/or data management programs (The Fujitsu version has TISP and Data Management, the IBM version has Communication Server and DFSMSDFP etc.) that will output return codes or reason codes. These codes will be output in HULFT's logs (reference below) and the SYSLOG, so please reference these.

(The above screen is an example of a receive error)