Why does my screen say 'check cables' when they look fine to me?

Q  When I turn on my desktop computer, I get a message on an otherwise blank screen that reads: ‘No connection. Check cables.’ The PC still seems to be working, as it’s making all the usual noises. The problem first began intermittently and now it is constant. All cables are connected correctly.
Thelma Burns

A  This is happening because for whatever reason, the video signal from the graphics card in the PC isn’t reaching the monitor. So, as far as the monitor is concerned, there is no PC connected, hence the ‘no connection’ message.

Identifying the cause will involve some trial and error, but there are a few things to check.

First, ensure that the monitor is set to display images from the correct input. You didn’t tell us the monitor’s make and model but most modern displays have analogue (VGA) and digital (DVI) connections, while many have HDMI inputs too.

It’s usually possible to switch between these connections using a button on the monitor’s front panel. If this has been set to display images from an input that has nothing connected to it, then the result would be an error message like the one you described.

Although the intermittent nature of the problem suggests that this probably isn’t the cause, it is the quickest thing to check and rule out – so refer to the monitor’s manual to figure out how to switch between inputs.

As you’re adamant that the cables are properly connected, the next things to look at are the plugs at either end of the cable that connect the graphics card to the PC (or just at one end, if the monitor has a ‘captive’ cable).

Old-style D-sub plugs (so called because their shape resembles the letter D) have several small pins that can be easily damaged with careless use, so check to see if one hasn’t been bent or snapped. The same applies to newer DVI plugs (which have a more rectangular shape), though these are much sturdier and more difficult to damage.

Otherwise, the cable itself may have failed somewhere along its length due to misuse or simply old age, so connecting a different cable will rule this out.

The next most likely culprit is the graphics card itself. If you’re able to, remove the PC’s case and check that the card is correctly seated in its socket: we recommend wearing an anti-static wrist-strap before venturing inside, though; and make sure the computer is switched off.

Now just apply firm thumb pressure along the length of the card’s top edge and ensure that any restraining clips are properly engaged.

If after all this the monitor continues to display the error message that leaves either a faulty monitor or a faulty graphics card as the cause.

Practically, the only way to find out which one isn’t working properly is to swap one or the other with one that’s known to be working – so perhaps borrow a monitor from a friend, as that will be easier than swapping out the graphics card.