Zanussi Chest Freezer - Does this sound like failing thermostat?

[T5 TDI]

About 3 months ago the freezer suddenly began warming up. I emptied it and defrosted/dried it all out. A few days later I tried it again and it began cooling down and was again working perfectly. I didn't trust it, so I put the outdoor thermometer from my weather station inside and set up notifications to my phone for anything above -15c. It had been working perfectly for months and hovering within half a degree of -18c overnight and only rising slightly when opened. Then (while we were away!) we started getting notifications and the graph showed that at 4.30 am it suddenly started warming again. I was surprised at the speed of the warming.

My question is- Is this behaviour (working perfectly for months then suddenly failing but then coming back to life again) normal for a failing thermostat? I believe the freezer is about 6 years old. This would probably be the extent of what I could fix myself.

 

[T5 TDI]

I've decided to just replace the thermostat as a test. I have it freed from the freezer but I can't work out how to separate the actual thermostat from the plastic panel. I'm hoping it doesn't come as one big expensive thing! Does anyone know a good place to buy a new one? I don't want to spend £100 on a Zanussi one just to throw it away if it doesn't fix it. The trouble is I can't test it because it's working perfectly now it's had a rest! It might be another 3 months before it stops again.

Thanks for any help. Here are some pics of the problem:

 

[JohnD]

How often do you defrost it?

Is it a "frost-free" freezer?

 

[Harry Bloomfield]

I didn't trust it, so I put the outdoor thermometer from my weather station inside and set up notifications to my phone for anything above -15c.

We have a new freezer, set at -20C and I have a gauge monitoring that. At times I do see that a little warmer than -15C. Our older one, before it failed, would always hold the temperature closer to the set temperature. I would suggest the larger variation is due to heat from auto-defrost and due to the use of an internal fan - it makes short excursions higher than the set temperature, but it's the temperature of the items stored in there, that matters.

 

[T5 TDI]

I don't defrost it that often. But after it completely stopped freezing the first time, we defrosted and dried it out fully. Six weeks later it failed again. (It's not a frost free freezer.) I have ordered a cheap thermostat to see if will keep going more than 6 weeks!

I was quite happy with it while it was working, it was only varying by a few tenths of a degree overnight (when it wasn't being opened). It was only at 4.30am on the fateful day in the graph above that it decided to give up completely. The compressor had stopped and it wouldn't start the next day. Several days later we tried again and off it went and chilled down as if nothing had happened, until the next episode six weeks later.

We have since bought a £10 bargain old freezer as a temporary stop gap but it's pretty bad. It's keeping up to temp but it's on more than it's off. I stuck the same temp sensor in it and this old wheezing thing (the top seal is poor) is varying by more than two degrees and working itself to death doing it.

Dodgy old £10 freezer

I managed to pop the thermostat control knob out and get to the nut so I'll be back in a few months to report on whether it cured it! Thanks for your replies.

 

[T5 TDI]

Back sooner than I thought! I'm struggling to get the new thermostat probe back in. It has a nice plastic tube to guide it and it slides into it perfectly but it comes to a sudden stop with a light clonk with about a foot of the probe still to go. The old probe is the same. As is the bit of stiff wire I tried. It feels like the end of the guide tube inside the freezer has moved slightly so the probe misses where it supposed to go. Has anyone else had this? Any tips I could try? There doesn't seem to be any way of getting at the freezer insulation to look at what is going on.

Edit- I'm wondering if this last foot simply goes into the foam, in which case dare I use my bit of wire in a drill chuck and forge a new path? I can hear that the clonk where the probe stops comes from about half way up the freezer or just over.