A technique for sequencing nucleic acids in an automated or semi-automated manner is disclosed. Sample arrays of a multitude of nucleic acid sites are processed in multiple cycles to add nucleotides to the material to be sequenced, detect the nucleotides added to sites, and to de-block the added nucleotides of blocking agents and tags used to identify the last added nucleotide. Multiple parameters of the system are monitored to enable diagnosis and correction of problems as they occur during sequencing of the samples. Quality control routines are run during sequencing to determine quality of samples, and quality of the data collected.