Does precipitation influence the UV spec reading?
The answer to the question as asked is that yes, any precipitate will affect the value that you are reading in your spectrophotometer. The basis of spectrophotometry is that light passes through a sample, and less light emerges than the amount which entered. The amount of light that has "disappeared" is used to deduce something about the solution in the cuvette. Light can be absorbed by chemical groups (this is the basis for e.g. DNA having an absorbance value) or it can be scattered by particulate material (this is the basis for measuring the cell density of a bacterial culture). If you are trying to make a measurement of the first type (absorbance) and you get a precipitate your reading will be distorted by the light scattering effect.

Precipitates are incompatible with spectrophotometry.

As the comments have noted: if you give more details of your protocol someone may be able to help. Phosphates are, of course, notorious for generating unwanted precipitates.