I don't know if you have the same Dimplex fireplace, but just had to pull ours out to fix. I called customer service with the symptoms, and they said if the heat fins that the fan blows across are covered with dust, etc. not enough airflow passes across them and the unit will automatically turn off the heat function, but the lights will still work. Remove and open the unit and find the heater unit. If very dusty, clean with vacuum, or light compressed air (gently)
If that fails to resolve you will probably need a new heat/fan assembly. I got lucky and ours still works better than ever.
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