How long does AC leak dye last?


Diamond Member
Dec 3, 2010
Last year I had my A/C unit serviced and it needed a top up of freon because it wasn't cooling properly. The technician also put in UV leak detection dye into the unit. The unit worked fine all summer and I didn't need to call the technician back. Last week I turned on the A/C for this first time since last year. Same problem as last year - unit was cooling but not very cold, and could not cool to 75 degrees with outdoor temps in the 80s. Will the UV dye still show where leaks are or will I need to get another UV dye test?


Platinum Member
Aug 17, 2006
Yes, it should work. The dye lasts for years - until it leaks out, of course. The tech will try to get the area dark and use a UV lamp to look for a yellowish-green stain somewhere along the system. But of course that means the leak must be somewhere you can see it. Some places - like the back surface of the condenser "rad" in front of your regular rad, or inside your heater where the evaporator core is - can't be seen. My leak (found last week) was visible on a suction pipe connecting to the expansion valve mounted on the engine side of the firewall. I have parts on order.