Fish – Is whitebait safe at 42c

With thin cuts of meat (like chicken breast or a steak), you want to make sure you put the thermometer in to the thin side of the meat so that the whole thermometer goes in to the meat, rather than putting it in through the "top" so that only a little of the thermometer is in the meat. If you're putting it in the top, you can get wildly inaccurate temperatures. Also make sure that the thermometer isn't touching a pan, exposed to air, or touching a bone. Any of those can have a negative effect.

Also, you only need to cook chicken to 74C / 165F. That may be part of the problem in your case.

You may also want to check that your thermometer is accurate. You can stick it in boiling water to check that it reads 100C / 212F (assuming of course it goes up that high).

Assuming the cod was the same quality as before and you de-salted the fish in the fridge for 24 to 48 hours, then I would also guess overcooking. Rendering the gelatin at 45°C (as suggested in the paper @yossarian uncovered) should prevent the fish from cooking and give enough time to get it into the oil. You can always cook the fish in a separate step after you made the pil pil.

Modern techniques for salting cod use less salt and create a product that will not be edible by future archeologists, so store in a dry cool place (tightly wrapped in the fridge).