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A transmission leak is easy to spot and miserable to locate. Fluid runs down the case, wicks along crossmembers, drips off the lowest point, and by the time it hits the floor it is nowhere near where it escaped. Wiping everything down and looking again works, sometimes, after a week of driving. A UV dye kit does the same job in one drive cycle and tells you the exact seal, gasket, or line that failed.
The concept is simple. You add a small amount of fluorescent dye to the transmission fluid, drive the vehicle to circulate it, then shine a UV light under the car. The dye glows bright green or yellow-green exactly at the leak point. No glow anywhere else. It removes the guesswork that gets people to replace a pan gasket when the real leak was the front seal a foot forward.
Two Things Have to Be Right
1. The Dye Must Be ATF-Compatible
This is the mistake that wrecks otherwise good detective work. Generic universal dye is formulated for oil or coolant and can be too thick, or chemically wrong, for automatic transmission fluid. Buy dye that is specifically labeled safe for automatic transmission fluid and power steering, or a true universal dye rated for ATF. Dose it correctly, typically a quarter to a half ounce for a full transmission. Overdosing does not make it glow brighter; it just wastes dye and can foam.
2. The UV Light Must Be Strong Enough
A weak keychain UV flashlight will not cut it under a car with grime and daylight bleeding in. You want a proper 390–395nm UV lamp with real output, ideally a high-LED-count light or a corded 100W-class inspection lamp for a shop. The included amber or yellow glasses are not a gimmick; they filter reflected UV so the fluorescing dye pops against the background. Use them.
What to Buy — 2026
Full Kit: Dye + UV Light + Glasses
For most people, the all-in-one kit is the right buy. You get an ATF-compatible dye bottle, a UV inspection light with the correct wavelength, and the amber glasses in one box. Look for kits that list ATF and power steering among the compatible fluids, and that include a rechargeable or high-output light rather than a dim button-cell unit. This is the fastest path from red puddle to a named part.
Browse UV dye leak detection kits
Dye Only: If You Already Own a UV Light
If you have a decent 395nm UV flashlight already, just buy the dye. A multi-application fluorescent dye bottle rated for ATF costs a fraction of a full kit and does dozens of vehicles. Confirm the label lists automatic transmission fluid before you dose it.
Check ATF leak detection dye
Upgrade: Corded Shop UV Inspection Lamp
If you chase leaks regularly or the fluorescing is faint on your daily driver, step up to a corded high-output UV inspection lamp. The extra output makes a marginal, weeping leak obvious where a pocket light shows nothing. Worth it for a home shop that does this more than once a year.
Check UV inspection lamps
How to Actually Use It — Step by Step
- Clean first. Degrease the underside of the transmission and wipe it dry. Old dry ATF residue does not fluoresce like fresh dyed fluid, but a filthy case makes everything glow dimly. Start clean.
- Add the dye. Pour the recommended dose into the fill tube or dipstick tube. On sealed transmissions with no dipstick, add it through the fill port when you top off.
- Drive to circulate. Fifteen to thirty minutes of normal driving, including some highway, gets the dye everywhere and pushes fluid past every seal under pressure and heat.
- Inspect in the dark. Put the vehicle on stands or a lift, kill the shop lights, put on the amber glasses, and sweep the UV light slowly across the pan rail, front seal area, cooler lines, output shaft seal, and the case seams.
- Follow the glow to its highest point. Fluid runs down. The actual leak is the topmost point of the glowing trail, not the drip at the bottom. That top point is your failed part.
Found the Leak? Get the Seal or Gasket — Ships Same-Day
Once the dye names the part, our companion shop gearboxinsider.com stocks pan gaskets, front and output shaft seals, cooler line fittings, and full reseal kits for the common GM, Ford, and Dodge units. Same-day ship before 2pm Eastern, free shipping on every order.
Shop Gearbox Insider →Common Leak Points the Dye Will Reveal
| Glow Location | Likely Source | Typical Fix Cost |
|---|---|---|
| Along the pan rail, all sides | Pan gasket or overfill | $40–$150 |
| Front, behind bell housing | Front/pump seal | $400–$1,200 (trans out) |
| Rear tailshaft | Output shaft seal | $100–$300 |
| At a metal-to-rubber fitting | Cooler line or fitting | $80–$300 |
| Top of case, wiring area | Sensor / connector seal | $60–$250 |
FAQ
Will UV dye harm my transmission?
Not if you use a dye rated for automatic transmission fluid and dose it correctly. It is a tiny concentration relative to the fluid volume and stays in the fluid harmlessly. Do not use coolant-only or fuel-system dye in ATF.
Do I have to drain the dye out afterward?
No. At the correct dose it is inert and stays in the fluid. It comes out at the next fluid service like anything else. It will not affect shift quality.
Can I use a regular blacklight flashlight?
A true 395nm UV flashlight works. Cheap "blacklight" novelty lights that are really just purple LEDs do not have the wavelength to make the dye fluoresce well. Buy a real UV light and wear the amber glasses.