er3 solution

er3 originally is a fan error. Your dryer expect reducing temperature on PTC heater, but in rooms and regions with hot air it might take a longer time than sunlu expecting. That's why it thinks thant your fan isn't blowing at all or damaged for some reason. Spacers let cold air from outside get in dryer → ptc heater cooling faster → no error. This mod does not decrease max temp but probably increases power consuption.

Also you need to buy screws longer than stock. 
Sunlu leg - 10mm gap (+10mm screw length)
Sunlu leg_15 - 15mm gap (+15mm screw length)

Mine S4 perfectly works with 10mm gap, if your room is really hot, try to use 15mm spacers.