Why does my fridge run cooler in summer than in winter?
My fridge was around 4°C during the winter but now we are approaching summer it has got warm here and the fridge is at -0.6°C.
What is going on?

 

The obvious answer is that the temperature sensor is being affected by the external temperature. Some radiated or conducted heat is making it think the refrigerator is too warm. Sometimes this can be fixed with a little wad of insulation in just the right place.

 

I think there's also a statistical answer. It will obviously run more in the summer, which makes it more likely that you'll look at the thermometer while it's running or recently ran, as opposed to when it's been sitting silent for a while. There may be stratification and other issues that cause artifacts in the temperature reading.

 

It's just that crossing zero C is a critical point making the difference between excellent performance and ruined food.
If food = frozen, move setting. If food is not frozen, reconsider how you are measuring.
I use a stainless steel thermometer in a stick of butter to average out fluctuations and make the thermometer hold still while I am reading it.

 

I think #12 may be right. I've noticed it doing this for quite a few years because things get frozen in summer.
I will investigate the temperature sensing. I seem to remember it is a copper pipe so probably liquid or gas expansion type.

[Edit] I've had it quite a while and it wasn't new then (it came with the house). So maybe it's time I bought a new one which would probably be a good bit more efficient.

 

I think #12 may be right.

Refrigeration was my day job for about 30 years.

Mucking about with tiny defects like seasonal variations can be very time consuming. The cause is usually bad design, but the cure is usually small and simple...right after you figure it out. In this case, adding thermal mass to the sensor bulb would be exactly the wrong thing to do. In fact, a failed sensor with too much hysteresis could be the entire problem. You want the sensor to be more sensitive to the inside temperature than the outside temperature. You might even re-route the sensor bulb to be entirely inside the refrigerator instead of partially buried in a wall cavity. Nobody can know unless they are looking at the problem, in person. The subtleties are just too many to describe.

The problem with assuming: newer = more efficient is that it is not necessarily true.
I don't remember any laws about refrigerator efficiency, but there have been several laws about air conditioner efficiency.
And remember, bigger is worse. A double door Amana with all the bells and whistles is an energy hog compared to a top-freezer 17 cubic foot with nothing more than automatic defrost.