System doesn't turn on, power supply good, no LEDs on board

I’m trying to debug a mid-2017 21.5” iMac that isn’t turning on. The power supply that was in the computer was bad (no 12v or 3v output), but after replacing it with a known-good unit I’m still not getting any signs of life. No fan spin, no chime, nothing. The diagnostic LEDs are also not lighting up, none of them. Is this a solvable problem?