I have been enjoying excellent results making yogurt in my cooler for a couple of years now, so you can imagine how dismayed I was to bomb the last two recipes in a row! They turned out runny - barely thicker than milk - and quite sour. We made smoothies out of them, so all was not lost, but it was a bit disheartening (especially since I was just telling my sister-in-law how foolproof my method seemed to be!)
I combed my memory for what I had done differently, but my technique, my milk, my starter were all the same. Finally I remembered that I had broken my old candy thermometer and bought a new one, just before I made that first batch of runny yogurt.
My suspicions aroused, I researched how to tell if your thermometer is accurate, and found this obvious and simple way to check: boil a pot of water and see if your thermometer reads 212 F (100 C). I tried this, and found that my new thermometer ("Accurite" brand - hah! It was neither right nor accurate!) was reading 230 F when the water boiled.
Now I could either remember every time that my thermometer reads 20 degrees off (so I would have to get my milk to 130 degrees, according to this thermometer, instead of 110 before I add the starter) or buy a new thermometer (which I will probably do - my old, accurate one was a "good cook" brand, which I can find locally and inexpensively). Knowing me, I'd forget, so I'll just take the $7 hit and buy a new one.