I had a new shower installed about a year ago,it has a single handle mixer valve, yesterday the water pressure was very low and today the same thing.

Water pressure is good in all the other faucets in the house, including the sink, and toilet in the same bathroom as the shower.

Is the problem in the mixer valve?

Thanks,
golfbum70

Is it low for both hot and cold? If so it may be the mixer but check your shower head for debris. Try removing the head completely to see iof it was the cause of low pressure. If not then disasssemble the valve cartridge. Click back here if you want assistance.