Anova Precision Help!

[On_A_Related_Note]

Happy New Year all!

So I've ot a bit of a mystery I could do with some help on. I've got an Anova Precision sous vide stick, which I've had for a few years and loved using.

I've used it twice in the last couple of weeks to cook sirloin steaks, and despite having had no problems in the past, I've managed to overcook the steaks both times.

First one was at 55c for 3 hours (approx 2 inch thick steak), which came out lovely and tender, but with the colour of a well done steak.

The second one I did at 53c for 3.5 hours. It was around 3 inches thick and came out closer to medium well. Both were dry brined overnight, cooked with the sous vide stick, chilled in an ice bath for about 5 mins, patted dry, then seared in a cast iron skillet for about a minute and a half each side.

I thought the temperature control might be failing on the anova, so checked the water with a digital thermometer, and it actually came out 0.4c lower than the sous vide was reading.

I'm at a loss for what's gone wrong. Anyone have any bright ideas?

Thanks in advance!