2015 iMac runs slow and fan running fast after replacing display.

I replaced it with a surplus display I had for a 21.5” iMac. I thought it was a retina display. I ran TGPro and, under Diagnostics, it returns an amber button next to Temperature Sensors with the following text: These sensors may not reworking properly: Unknown name.

Does anyone have any ideas on how to troubleshoot this further. FYI: I do not have the original LCD display.

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@burningrio of course you already tried the usual

Resetting the System Management Controller (SMC)

The System Management Controller (SMC) is a chip on the logic board that controls all power functions. If the computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, or fan noise issues, resetting SMC may resolve it.

For more information and instructions to reset the SMC on different computer models, refer to article HT201295: Reset the System Management Controller (SMC) on your Mac.

Note for iMacs: If the power button is pressed while inserting the power cord, the iMac will enter a mode in which the fans run at full speed. For more information, refer to article HT204463: iMac: Fans run at full speed after computer turns on.

Resetting Non-Volatile RAM (NVRAM)

NVRAM stores certain system and device settings in a location that macOS can access quickly. Exactly which settings are stored in the computer’s NVRAM varies depending on the type of computer as well as the types of devices and drives connected. To reset NVRAM:

1. Shut down the computer.

2. Locate the following keys on the keyboard: Command, Option (Alt), P, and R. You will need to hold these keys down simultaneously in Step 4.

3. Press power button.

4. Immediately press and hold Command-Option-P-R keys.

Important: You must press this key combination before the gray screen appears.

5. Hold down keys until computer restarts, and you hear startup chime a second time.

Note: For MacBook Pro (Late 2016 and 2017) and MacBook (Retina, 12-inch, 2017), hold down keys for at least 20 seconds. There is no startup chime.

6. Release keys.

Note: After resetting NVRAM, you might need to reconfigure settings for speaker volume, screen resolution, startup disk selection, and time zone information.

Starting Up in Safe Mode

Safe Mode (sometimes called Safe Boot) is a way to start up a Mac so that it performs certain checks and prevents some software from automatically loading or opening. These changes can help resolve or isolate certain issues on the startup disk.

Follow these steps to start up into Safe Mode:

1. Be sure the computer is shut down.

2. Press the power button.

3. Press and hold the Shift key.

Note: The Shift key should be pressed as soon as possible after the power button is pressed.

4. Release the Shift key when you see the Apple logo appear on the screen. After the Apple logo appears, it may take longer than usual to reach the login

5. To leave Safe Mode, restart the computer without pressing any keys during startup.

Since you do not have the original display, maybe you can give us all the numbers of the label from the current display. Ultimately I would start by checking the LCD temp sensor Apple part number 923-0280.