To enable or change the logging threshold, you need to edit your config.php file. This file is located in the application/config/ folder. Look for the line with $config['log_threshold'].

You can enable error logging by setting a threshold over zero. The threshold determines what gets logged. Threshold options are:

0 = Disables logging, Error logging TURNED OFF
1 = Error Messages (including PHP errors)
2 = Debug Messages
3 = Informational Messages
4 = All Messages

You can also pass an array with threshold levels to show individual error types

array(2) = Debug Messages, without Error Messages

For a live site you'll usually only enable Errors (1) to be logged otherwise your log files will fill up very fast.

To be able to figure out an error or a bug, change the logging level to 4. AFter that, reproduce the error and look in the log file to see if you can find anything that is related to the problem you are experiencing. Posting the log when you create an issue, is always helpful.