The C5 sensor is a particle detector similar to a smoke detector, which works with the help of a photoelectric sensor, that can be triggered by dust or other small particles in the air of 2.5 microns or larger.
When the keypad shows C5 on display, the particle detector is alerting the user that there are too many particles in the air.
To rectify, you can either reset the sauna at the outlet or disconnect the Molex connector on top of the sauna – see the PDF attached to this article on how to do this. To access the connector you can reach it either through the internal speaker or through the exterior ceiling cavity.
To reset, simply unplug the sauna for 60 seconds, let residual power drain out and plug back in.