Intel-based Mac computers use a combination of tones and blinking LEDs to display POST error codes. Some newer Macintosh models only use startup tones to indicate error conditions.
If your Mac detects no Random Access Memory (RAM)—or if the RAM installed doesn't meet specifications—the display remains black but the power LED on the front of the computer (if present) blinks once per second to signal the error. This error condition might be due to physically damaged RAM, incorrect RAM, or not having RAM installed.
Some RAM might appear to pass the POST but still can't be used by the operating system. In this case, your Mac shows a gray display, you hear three tones, and the power LED on the front of the computer (if present) blinks three times, pauses, and repeats until the computer is turned off.
To fix these issues, first re-seat the memory and test the computer. If the memory fails the POST test again, try memory that's been verified to work correctly on another system (i.e., "known-good") or new memory.