DIAGNOSIS The 'burn' alert is the unit's smoke detection kicking in prematurely. It's almost always caused by excess smoke from oils or drippings hitting the heating element, not from your food actually being charred.
FIX 1) For fatty foods like bacon or burgers, you are likely overfilling the basket. Use less food to allow for proper air flow and to prevent excessive dripping. 2) Always place a layer of foil or the provided tray at the bottom of the pot under the crisper plate for extra-greasy items to catch drips. 3) Make sure you're not using aerosol cooking sprays with lecithin, which can burn and smoke at high heat. Use an oil mister with pure oil instead.
IF THAT DIDN'T WORK A very dirty heating element or inner chamber can also cause smoke from old residue. Give the removable inner chamber and the heating element a thorough clean as described in the cleaning question.
HEADS UP The smoke sensor is sensitive and located near the top vent. Cooking in an area with poor ventilation (like under a low cabinet) can trap steam and trigger it. Give the unit some overhead clearance.