Colored bar/flickering on iMac late 2013: what could be the causes?

[scorzese]

Good morning everyone!
Last night this bar appeared:

Today, while, through the same iMac, I was browsing the internet about the prices of the new 24-inch, everything returned normal (it could be jealousy... ?).

What could have caused that bar? Problem with ... RAM? GPU?
The Mac is old, it is turning 10 next year, and has been really used almost every day over the years.

Also a year and a half ago I change HDD to SSD. Could there have been hardware corruption during the installation?

 

[Andrea Filippini]

Good morning everyone!
Last night this bar appeared:


View attachment 1994881
View attachment 1994882

Today, while, through the same iMac, I was browsing the internet about the prices of the new 24-inch, everything returned normal (it could be jealousy... ?).

View attachment 1994883

What could have caused that bar? Problem with ... RAM? GPU?
The Mac is old, it is turning 10 next year, and has been really used almost every day over the years.

Also a year and a half ago I change HDD to SSD. Could there have been hardware corruption during the installation?

It can be the GPU or the motherboard.
Have you tried to detach and hang up the power cable?

 

[scorzese]

No, he settled on his own.

 

[Andrea Filippini]

No, he settled on his own.

Try PRAM and SMC reset.
If the problem persists, contact a third party supplier.
It can be also a thermal paste issue.

 

[Nguyen Duc Hieu]

What could have caused that bar? Problem with ... RAM? GPU?
The Mac is old, it is turning 10 next year, and has been really used almost every day over the years.

Also a year and a half ago I change HDD to SSD. Could there have been hardware corruption during the installation?

It's more likely an issue with the LCD panel rather than the computer part.
Probable cause:
LCD panel
LCD panel T-con board (the board attached to the back of the LCD panel, covered in tape)
Bad connection of the LVDS cable between the logic board and the LCD T-con board.

In any case, a normal repair shop will offer you to replace the whole LCD panel set.
It's a little difficult to troubleshoot and fix at component level in this case.
(But for the TV repair shops in my country, this is one of their daily jobs, not 100% fixable anyway)