Using an old iMac as second monitor

The new iMac does not support using the old iMac as a display.

carefully read > Use your iMac as a display with target display mode - Apple Support

If you want to use a second display for the new iMac..?

Then you will need to get and use an after-market display.

see > Connect an external display to your iMac - Apple Support

You’ll need to identify the years for both the old and the new iMac.

Details: Use your iMac as a display with target display mode - Apple Support

If the new Mac is 2020 or newer, it won’t work.

Tipper576 wrote:

I have an old imac running macos sierra and want to use as a display monitor only . i cannot get my new imac to see it. What am i doing wrong? I have the thunderbolt cable running from the old to the new via an adapter. Help:)

Even if your old iMac supports Target Display Mode, your new iMac doesn't support using ANY iMac as a Target Display. See the bad news here: Use your iMac as a display with target display mode - Apple Support .

If the old iMac was released in 2019 or later, and is running Monterey or later, you could configure it as an AirPlay Receiver for AirPlay to Mac. That's a lower-quality connection method than a cable to a monitor. Odds are that your old iMac is too old to act as a second screen in this fashion.

Hi,

Following article could help your issue.

Use your iMac as a display with target display mode - Apple Support