The C13 parameter sets regen strength, but there may be a hardware switch to enable it. On mine, there is an EABS jumper. It's a pair of grey wires on my controller with a 2 pin connector. Comes with a two pin plug to complete the circuit, which enables automatic regen. If you have this circuit on your controller, and it is plugged in, unplug it and see what happens.

My geared motors can't do regen anyway, but I cut the grey wires off to clean up clutter. Also removed the wires for lights (they call it lamp/speaker), cruise control, and speed limiter.