If the hard drive is disconnected and you already checked that the fan is working, overheating is probably not the reason why it’s in a constant restart loop.
Only other thing I could think of is to make sure you didn’t over tighten the long fan screw and T6 logic board screw. if you have an adjustable torque driver, it must be set to 2.0 in. lb. Over tightening could bend the logic board, possibly damaging surface mount components.
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I just checked and this model has a flash storage, it could be loose, reseat or temporarily disconnect to diagnose if this is causing the constant reboot.
If the hard drive is disconnected and you already checked that the fan is working, overheating is probably not the reason why it’s in a constant restart loop.
Only other thing I could think of is to make sure you didn’t over tighten the long fan screw and T6 logic board screw. if you have an adjustable torque driver, it must be set to 2.0 in. lb. Over tightening could bend the logic board, possibly damaging surface mount components.