Last week I told you that printer error codes mean almost nothing.

This week I am telling you the opposite about your computer.

A printer code is a filing number that helps a support department, not you. A blinking light on a motherboard is a genuine message, sent on purpose, because it is the only way the machine has left to speak.

Before Windows loads, before the screen shows anything at all, your computer checks itself. Memory, processor, drive, graphics. It does this in a fixed order and it takes a couple of seconds. If any of those checks fail, it stops right there. And at that moment it has no operating system, no drivers and no way to put a single word on a monitor.

So it uses what it has. A light. A beep. A pattern.

A lit DRAM light on a desktop motherboard means the memory check failed. Sometimes that is a dead stick of memory. Far more often, on a machine that got moved or bumped or worked on, it is a stick that has come slightly loose in its slot. It looks seated. It is not.

Beep codes are older and still around. One long beep and two short ones usually points at graphics. Repeating short beeps often means memory. The pattern means something slightly different depending on who made the board, which is annoying, but count the beeps and write them down. That is real and useful information.

A CPU light or a BOOT light narrows it further and those are worth reporting exactly as you saw them.

No lights, no beeps, but fans spinning. That is its own signal and it is usually the least good news on this list.

Now the boring causes, which are also the common ones.

The monitor is on the wrong input. Check this first, every time. It costs ten seconds. Leaving the computer on, turn off the monitor and unplug it from power. Wait a minute, plug the monitor back in and turn it back on. Autoscan should connect it to the correct source.

The cable is in the wrong port. If the desktop has a graphics card, the cable goes in the card, not the row of ports on the motherboard above it. People reconnect after a move and mistakenly connect to the wrong port.

The memory is loose. On a desktop, if you are comfortable opening a case, pressing the sticks firmly back into their slots is a genuine two minute fix. Power off and unplug first. On a laptop this does not apply, because the memory is usually soldered to the board and there is nothing to reseat.

The monitor or cable died and the computer is fine. It happens more than people expect. Try a different cable (most are now HDMI, the same as your TV and everything that plugs into it). Borrow another screen, or connect to your TV.

What not to do: do not start pulling parts out to test them one at a time unless you know what you are doing. Memory slots have delicate contacts and a graphics card has a retention clip that is easy to snap in the wrong direction. The savings from a self-diagnosis disappear the moment you bend a pin.

So here is the one thing I would ask of you.

Before you unplug anything, before you move it, before you carry it out to the car, look at the machine and write down what it is doing. Which light is on. How many beeps and in what pattern. Whether the fans ran at full speed or normally.

That one line of notes is worth more than any description of the symptom, and it regularly cuts a diagnostic in half. Bring it with the computer.

Tomorrow: the machine that starts perfectly well, and starts wrong.

Mike