Have seen that mine is not an uncommon problem.
THE CARD: Sapphire Radeon RX 580 Nitro, purchased in Nov. 2017.
THE PROBLEM: As is well known, this card keeps cold (fan=0) in normal internet/office use.
It only gets warm during games.
At times, during a game, the card internal temperature sensor stops working (mostly freezing at 20ºC):
accordingly, the fans also stop and the card starts heating.
Luckily I have a separate fan controller with a sensor touching one of the card's cooling pipes.
The first time was back in November (card one year old) and I was a few minutes away from the PC: when I was back the fan controller alarm was on and the temperature had reached 103ºC!
I immediately stopped the game, the card cooled down and did not suffer any damage.
THE MITIGATION: My temporary solution is to set via TriXX the fan at a fixed speed (25% is enough for my games).
However, this is noisy and not good for the long-term health of the fans: this card is not meant to be used this way.
In most cases, after the card cools down, with a PC restart its temperature sensor works fine again, and I can with TriXX reset the fan to either Automatic or Custom.
I am not happy.
FREQUENCY. It may not happen again for weeks, even when gaming, and suddenly it happens again, perhaps twice or more per week.
CAUSE. The problem is surely hardware, not software-related. I tried re-installing, logging-out-in from Windows, Device Manager, nothing works.
I have also used GPU-Z and the GPU Meter sidebar: when the card sensor freezes, both report a constant 20º temperature reading.
I suspect that this fault is in the electronics and there is nothing to do except buying a new one.
QUESTION: However, I am now going to open my tower case for cleaning.
Is there anything I could do, such as cleaning the circuitry where the sensor is?
I am a retired PC technician so I am at ease with these things.
(formerly Seattle PC Service, Ireland, now in Italy)