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Technics SU-X920D abnormally hot

Hello everyone.

We recently got this Technics SU-X920D digital amp into our workshop.
The problem is that the top cover becomes extremely hot under normal workload. It does have a working cooling fan, however it only kicks in when the power amp is running at heavy load. (Like when the volume is set to about -30dB or at loud bass kicks)
After about an hour of testing with normal volume settings, the top gets so hot, it almost burns to touch it. We also got an SU-X520D for reference, this one doesn't get nearly as hot as the X920, plus the fan kicks in at much lower loads and temps.
I already refreshed the solder points on the power amp and replaced the thermal paste. Did not help at all.
Also, the customer says that this problem is fairly new. The amp did not get as hot before, so I'm guessing these temperatures are not normal.

Could the temperature sensor be broken? Or what else could be the problem?

Thanks a lot in advance.