Quite simply, this product does not work. it will not accurately test tire pressure on a presta valve. I even carried it to my dealer to see if A) his tech guy could get it to work, and B) warn him to never purchase one. I am as disappointed with this product as I have ever been with a product. Especially since I paid extra shipping to get it sooner. Never, ever purchase this product. Unless you are a glutton for frustration.

I purchased one of these and agree with the other buyers of the product. The presta gauge does not work with 90% of all the presta valves the gauge has been used with in my inventory of bikes. Purchase a better product. This one is junk.

I have had a similar experience to the reviewer who says it works one 1 out of 5 times. Except that I've tried it about 10 times on two different brands of Presta tubes and have never gotten a reading (although sometimes I do get the 2.0 PSI rating). I suspect that it is not compatible with all brands of Presta tubes, although it certainly should be! I gave it 2 stars instead of one because Slime's customer service department was VERY helpful. They offered to exchange/refund the gauge if Amazon wouldn't. I got a replacement from Amazon with the exact same issue...I am contacting an online bicycle parts store and they said it is a new item and are offering to take one off the shelf, try it out and tell me what they find. If anything useful surfaces I will report it here.

this thing works about every 5th time (which is how i know that i'm not just using things incorrectly). the other 4 times it seems to just not push the stem down no matter how hard i press it on the valve, or how i play around with the locknut. so it just reads 0 or some very low (e.g., 2psi) pressure.