When encountering a “could not connect to server” error in PostgreSQL, it’s essential to systematically diagnose the potential issues. First, verify that the PostgreSQL server is running by checking the service status using commands such as `systemctl status postgresql` or `pg_ctl status`, depending on your system. If the server is not active, ensure that it is started with `systemctl start postgresql` or the corresponding method for your OS. Additionally, ensure that the connection parameters—host, port, username, and database name—are correctly specified. The default port for PostgreSQL is 5432, and if you’re connecting to a remote server, ensure that the host IP is reachable and not blocked by a firewall.

Next, inspect the PostgreSQL configuration files, primarily `pg_hba.conf`, to confirm that the client IPs are permitted to connect. This file dictates client authentication and can restrict access based on the client’s address. Ensure that the listen addresses in `postgresql.conf` are set correctly; often, this is configured to listen on localhost, but for remote connections, it should be set to either `’*’` or the specific IP address of the server. Lastly, check your PostgreSQL logs for any error messages that may provide further insight; they are typically located in the `pg_log` directory under the data directory of your PostgreSQL installation. Analyzing these logs will often reveal the root cause of the connection failure.