Diagnosis
The projector is detecting an internal temperature problem, usually because it's overheating or the sensor is faulty.
How to Fix It
Power it down completely and unplug it. Let it cool for at least 30 minutes in a well-ventilated spot. Check that nothing is blocking the vents on the bottom and back. Plug it back in and try again. This solves it 90% of the time.
If That Didn't Work
The internal temperature sensor might be stuck. Do a full power cycle: unplug the power adapter from both the wall AND the projector for 5 minutes. Reconnect everything firmly and power on.
Heads Up
Never run it on a soft surface like a bed or pillow—the intake vents on the bottom will get smothered and trigger this error immediately.