How to Add Dye to an AC System to Find and Fix Leaks

Co-authored by Ed Beery and Eric McClure

Last Updated: June 28, 2022 References

If the air conditioner in your vehicle is blowing hot air out when you turn it on, you may have a refrigerant leak. Since your air conditioning system is pretty big, finding the source of the leak can be difficult. This is where injecting UV dye comes in! By using a manifold gauge to inject ultraviolet dye into your AC system, you’ll have a much easier time spotting the leak by looking for spots where the fluorescent color leaks out. Keep in mind, this is not a beginner-level process, and you’re better off taking the vehicle to a mechanic if you don’t have experience working on your vehicle’s AC system.

Question 1 of 9:

How do I know if my AC is leaking refrigerant?

If your AC blows hot air and you recently serviced it, it’s leaking. It’s totally normal for the refrigerant to run out over time, so don’t assume you’ve got a leak on your hands if your AC starts blowing hot air out and you haven’t had it serviced recently.^{[1]
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Research source} However, if you’ve had your AC system recharged or serviced and it starts blowing hot air again after a few weeks or less, you’ve almost certainly got a leak on your hands.^{[2]
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- If your vehicle is blowing hot air for the first time in a long time, try refilling the refrigerant first. You could simply need some fresh refrigerant.

Question 2 of 9:

Can I test and add my own refrigerant to my AC?

You can, but it’s a tall order if you aren’t a gearhead. Your vehicle’s AC system is fairly complex, and it can be hard to figure out which parts need replacing or fixing if you do have a leak.^{[3]
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Research source} If you’re familiar with how your vehicle’s AC system works, how a manifold gauge works, and what kind of refrigerant your vehicle needs, you can reasonably do this on your own.^{[4]
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- It’s not that expensive to get an AC leak repaired by a pro. It may not be worth your time or money if you need to buy a manifold gauge just for this.^{[5]
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- If your vehicle was made before 1994, it probably uses Freon, not modern refrigerant. Since Freon is a toxic substance and you can’t buy the version of Freon your vehicle needs without being a licensed mechanic, you’ll have to take it to a shop.^{[6]
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- If you’re thinking about working on your home AC system, don’t. Just call a technician. The results of DIY central air repair can be disastrous.^{[7]
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Question 3 of 9:

What do I use to add the UV dye?

Hook up an AC manifold gauge to inject the fluorescent dye. Turn your vehicle off. Pop the hood and hang a manifold gauge from the hook at the top. Take the open end of the low pressure line (which is blue) and screw it on to the low pressure port in your engine bay. Connect the high pressure line (which is red) to the high pressure port. Use the knobs on top of each coupling line to tighten the connections and secure them.^{[8]
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- The location of your pressure ports is going to be unique depending on your make and model. If you can’t find the ports, look for small tubes with caps labelled “H” and “L.” Take the caps off and hook your lines up to these ports.
- The high pressure line and low pressure lines are different sizes. If you don’t have labels on your pressure ports and your manifold lines aren’t latching on, you probably have them backwards.^{[9]
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Question 4 of 9:

Where does the UV dye actually go in?

You pour a few drops of UV dye into the yellow line on the manifold gauge. The yellow line, which usually hooks up to a compressor or vacuum pump and hangs from the center of the manifold gauge, will feed the dye into your AC unit. Pull the yellow line out away from your vehicle to avoid spilling UV dye all over your engine bay. Point the open end of the line up and carefully pour a little bit of dye into opening.^{[10]
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- Wear gloves to keep the dye off of your hands.
- Wipe the nozzle off to remove any minor splatters. The dye isn’t dangerous or anything, but any excess dye will make it harder to find any leaks.

Question 5 of 9:

How do I send the UV dye through the AC lines?

Hook that yellow line up to a can of refrigerant and start the car. Make sure the red and blue knobs on your manifold gauge are closed all the way. Get a can of refrigerant and attach the yellow nozzle to the valve on the can. Twist the knob on top of the refrigerant all the way down to puncture and open the can, and then open that knob up to send refrigerant through the lines. Put on protective eyewear and bleed the manifold gauge before opening the high and low pressure lines. Start your vehicle to send the dye through.^{[11]
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- You must bleed the yellow line before starting the vehicle. To do this, hold the manifold gauge away from the engine bay and use a flathead screwdriver or key to push the valve stem where the yellow tube meets the body of the gauge. Keep your eyes away from the gauge and release the pressure you’re putting on the bleed valve once liquid starts to come out.
- If the high and low pressure lines aren’t open, the refrigerant won’t travel through the AC lines. Just turn the knobs on your manifold gauge counterclockwise to open them up.
- Refrigerant is not universal, so consult your owner’s manual to see what kind of refrigerant you need to refill your vehicle.

Question 6 of 9:

How do I find the leak once I've injected the UV dye?

Grab a UV flashlight and let the vehicle run. Turn the light on and shine it in your engine bay. You may need to do a little searching since the AC system runs the entire length of the engine bay. If there is a leak though, it should light up immediately whenever you shine the light on it. If you don’t see any dye in your engine, look underneath the vehicle. If you still can’t find it after 10 minutes or so, you probably don’t have a leak.^{[12]
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- If you have to ask yourself, “Is this the UV dye?” the answer is almost certainly, “No.” The UV dye will be exceptionally bright and obvious.
- If you’re working outside, wait for it to get a little darker out. If you’re in a garage, turn the lights off. It can be hard to spot the leaks if it’s super bright.
- A regular blue or violet lightbulb will also cause the dye to light up so long as the bulbs are LED.

Question 7 of 9:

Is UV dye bad for an AC system?

No, so long as the UV dye is free of co-solvents. A standard UV dye doesn’t actually do anything—it’s an inert substance that functions basically like food coloring. However, you should use a dye that doesn’t contain any co-solvents. Co-solvents can damage aluminum, metals, and plastic.^{[13]
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- The good news is that almost nobody manufactures UV dyes with co-solvents in them anymore. Unless you have 10-year-old bottle of UV dye, there probably aren’t any co-solvents in there.
- Common co-solvents include Aromatic 200 and NMP (N-methylprrolidone). If the bottle of dye says “co-solvent free,” you’re good to go.

Question 8 of 9:

What color is AC dye?

UV dye is often fluorescent yellow, bright blue, or orange. However, the dye won’t actually shine that color unless you shine a UV or LED flashlight on it.^{[14]
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Research source} It may also be kind of difficult to spot the UV dye if you’re working in the sun or you have a ton of lights on in your garage, so do this when it’s darker out or turn the lights off.^{[15]
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- There are multi-colored UV testing kits that come with a bunch of different colors if there’s something specific you’re looking for.^{[16]
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Question 9 of 9:

What are the disadvantages of using UV dyes to find leaks?

There aren’t any serious disadvantages, but it can be a pain to clean up. The UV dye is extremely noticeable and bright, which is great when you’re looking for leaks. Unfortunately, the dye tends to get all over your engine bay once it starts sputtering out. In most cases, a damp rag will clean the dye up, but you can also mineral spirits, brake cleaner fluid, or specialized UV dye cleaner to remove the dye when you’re done.^{[17]
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- UV dye injection is considered the most accurate way to identify AC leaks by many mechanics. It’s basically foolproof since it’s impossible to not notice the dye if you do have a leak.^{[18]
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Warnings

If you aren’t mechanically inclined and you don’t have any experience working on AC lines or using a manifold gauge, you’re better off taking your vehicle to a mechanic. This is not an easy process, and diagnosing AC leaks can be kind of tricky.^{[20]
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UV dye injection is also used by HVAC technicians to diagnose leaks in central air units. Unfortunately, you cannot do this yourself. HVAC refrigeration lines contain Freon, which is toxic. You also can’t fidget around with a central air system if you aren’t a qualified technician for safety reasons.^{[21]
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