Dexter's reply is pretty extensive... regarding the idea of dirt on the lens, try a BLAST of canned air or something like that (don't touch it with your finger, even if you can see it)

Unfortunately, heat ruins these components often... and thus have to be replaced. Do you have the air vents covered, or do they need cleaning? It may have overheated... which might correct itself after cleaning... or you might have to replace the unit. (Depending upon the initial cost of these items, one should always consider a good warranty replacement agreement.)

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