Anyone who uses OpenCart store with large filter list knows that default filter logic might be a bit strange and a unexpected. For example, if you have filter groups Color and Size, and customer selects Color->Red, and Size->5, OpenCart shows all products that have red color, and all products that have size 5 at the same time. Instead of showing all red pruducts that are size 5.
In other words, default OpenCart does not combine filters with each other. So you may need an extension that can change that behaviour.
Fix OpenCart filter with QS Better Filter Logic
QS Better Filter Logic is a free OpenCart extension that changes previously described logic. It was built for OpenCart v1.5.6 series but it still works on the latest (v18.104.22.168) OpenCart – filter database queries did not change from that period. Everything you need in your system to make it work is vQmod installed.
You can get the QS-Better-Filter-Logic extension in OpenCart extension store here.