If your wall heater is cycling on and off every 15 seconds, and you are controlling your heater with a digital thermostat, then likely you need to update one of the thermostat settings.
If the heater does not turn on, it may have reached abnormal temperatures. If the manual reset trips (which turns the heater off), the heater probably needs to be cleaned or it’s blocked.
Make sure the voltage supply matches the heater rating and that the heater is clean. If it continues to run slowly, the motor and/or heater may need to be serviced or replaced.
On initial start up, your heater may smell. This is not dangerous, and is due to the manufacturing process. It typically goes away after the heater runs for several hours.