V70 air vents

[BLD]

My V70 (2004) with Climate Control has suddenly a marked difference in temperatures coming out of each of the dashboard vents, despite the controls being set to the same temperature. The driver side is much hotter and I can't get it as cold as the passenger side. This has only just happened and I don't know if there are sensors somewhere which are perhaps not working or if the problem may be more sinister.