Cycle times too long

It seems that lately, the dishwasher takes longer and longer to finish. The manual shows 84 minutes for a normal wash, but it's gone over 2 hours. At times I'll open the door and close it, that seems to get it unstuck. I read that there's an optical sensor, and if it doesn't like the water color, it can extend run times. Could it be getting dirty? If so, any idea where it is to check it? Would running a diagnostic cycle to re-calibrate help?

Thanks, Carl