You press the power button and your PC won't boot up. What do you do next? Don't write it off just yet.

The first thing to do when faced with a PC that won't start up is remain calm. They're modular devices, and it's likely that a single component among many is the culprit. Fixing or replacing that component should bring your machine back to life. The trick is finding out which one it is.

We recommend a systematic approach to the problem, starting with observation, moving on to elimination and ending in diagnosis and treatment. Don't worry, you won't need a degree in engineering to fix a poorly PC. The process will probably be much easier than you expect.

We'll take you through a troubleshooting procedure that should help you fix the majority of near-fatal hardware faults. In many cases, you won't need a great deal of technical know-how to get your PC working again.

PC power problems

There are many reasons why your computer might fail to start. Let's have a look at the possibilities, one probable scenario at a time. The first sounds like the worst, but could actually be one of the easiest to diagnose.

You switch on your computer and nothing happens. No lights, no fans, no drives whirring into life and, crucially, no POST (Power On System Test) beep. Somewhere in the chain, power isn't getting through.

Work your way through this checklist. Unplug all peripherals except the keyboard, mouse and monitor, and make sure these are connected correctly. Check the obvious possibilities: make sure that your power cable is fitted correctly, the mains is switched on at the plug and the power switch is on at your computer.

If you're using an extension cord or multiblock, take those out of the chain and plug your PC's power cable directly into the mains socket. You should also try a spare kettle lead if you have one, in case a break in the cable is responsible.

fuse box

THE BASICS: The PC itself may not be at fault. Check your fusebox first, along with fuses in multiblocks and plugs

Go to your main fuse box and check that all the fuses are still working. Check and replace the fuse in your computer's plug. Some PCs have a fuse in place before the Power Supply Unit, accessible near the power socket - you should replace that too. Only if you've eliminated all these possible weak links in the chain and your computer still refuses to power on should you move to the next stage.

The power supply

Problems with internal cabling are a common cause of power-on failure. Expansion caused by heat, movement in transit or a good old bump can dislodge connectors and cut power to your machine.

Open your PC case and make sure that power cables, especially those connected to the motherboard, are firmly seated and in place. Also pay close attention to the cable that connects the power button on the front of the case to the motherboard. If you're not sure which one that is, check your motherboard manual.

component checking

PRESS FIRMLY: Unseated cables and cards can cause a host of problems, from POST errors to a machine that won't power on at all

When you've done that, make sure your cards and memory are firmly seated too. Any dislodged cards could break the circuit in your machine, stopping it dead.

Next, check the heatsink and fan protecting your CPU. Are they aligned correctly and securely? Check that the feet securing the fan to the motherboard are fixed and solid. Your machine may not start at all without a properly positioned and fitted heatsink.

The diagnosis

If, after all this tweaking and testing, your machine is still failing to power up correctly, you're looking at four potential issues. If you're lucky it'll be the PSU (power supply unit) or the power button. Why is that lucky? Because these issues are both fairly easy and cheap to rectify.

There are two ways to verify a failed PSU. The existing PSU can be swapped out and another one tried in its stead. It's best if you can try one from an old PC you no longer use, to avoid unnecessary expense.

PSU tester

PSU TESTER: Using a PSU tester will enable you to determine if a faulty power supply is the problem

The other approach is to buy a dedicated PSU tester. Maplin sells one for as little as £20, but considering a new 750-watt PSU costs around £30, it's only worth buying a specialised tester if you expect to continue testing power supply units in future.

If a replacement PSU doesn't work, you might still have a power button problem. Again, cannibalising an old PC may be the best way to test your theory. The alternative is to search for and buy an ATX power switch on eBay. This should cost £3-5.

If you have an itch to confirm your diagnosis first, Maplin stocks a tool you can use to test the switch for £5. Visit the site and search for 'Domestic multi meter'. If the switch is working, you'll be able to pass a current through it.

If you've eliminated all the possibilities above and your machine still refuses to power on, then we're afraid that - finally - you're entering dead computer territory. It's likely that you have a fried motherboard or CPU, and a replacement - or a whole new computer - may be required.

Listen to the beep

You might encounter slightly different behaviour when your machine turns on. It may power up, beep, then hang indefinitely and seem to do nothing more. Careful diagnostics should be able to help you track down the component.

BIOS tester

MOBO TESTER: You can make diagnosing POST errors easier by investing in a motherboard testing card

The first clue will come from that start-up beep. We referred to the beep earlier as the Power On Self Test or POST beep. When your motherboard is powered up, it runs a diagnostic of the motherboard itself and the attached hardware. If all is well, you should hear a single short beep and your hard drive will whirr into life, booting the operating system.

If the POST encounters a problem, you'll hear a different sequence of beeps.

New hardware

If you hear an unexpected beep sequence, it might be caused by several different things. If you've recently added new hardware to your PC - memory or a new video card, for example - there's a high likelihood that could be the problem. With the computer powered down, open the case and reseat all cables and cards. Any of them could have been nudged slightly or dislodged while you were fitting that new component.

If you've ruled that out, try removing the new hardware component. If it's a component that the PC needs to run, like a video card, replace it with the old, previously working component. Close the case and switch it on. Is it still not working?

It's likely that a component in your system is damaged. Time to research those beeps. Make a note of your BIOS manufacturer's name and note down the beep sequence. You may hear more than one short beep, or you may hear a more complex sequence. For example, a failure in the video system with Award BIOS may generate one long and two short beeps.

You can find a comprehensive list of POST beep codes at BIOS Central. It'll help you narrow down the source of the error and decide whether to fix your machine or fling it in a skip.

Graphics card failure

Video cards are common failure points in modern systems, and when one goes it can make your system appear to be dead when it's working perfectly well. With powerful GPUs on board and many relying on passive cooling, they're prone to overheating or blowing if you experience a power surge.

Diagnosing the POST beep will help you reach a swift conclusion, but there are other clues that can tell you if the video card is what's on the fritz. In many cases, your machine will complete its boot sequence and load the operating system. It could be waiting politely for you to enter your username and password.

2nd pc

HAND-ME-DOWNS: Most people have an old computer collecting dust, so re-use the components you can either for testing or for a new rig

The problem is, you won't know that if the video's not working. Signs to look for are a POST beep that indicates video failure, drives powering up as normal, drive lights and power lights on the front of the system. We've had circumstances where, even though the video wasn't working, we were able to connect an affected computer to a network to retrieve data. This is handy if you need to access files quickly.

The cure for such a problem is simple. Install a new video card. It doesn't have to be a fancy one to get the machine going again, so feel free to pull one out of an old PC if you can find one with compatible components.

Drive failures are among the most frustrating computer problems. They can come back to haunt you again and again if you don't take the right step first time. With most drive failures, the boot sequence will complete. You may even get a clear POST test, with a single beep.

After that, the OS loading sequence will help you determine how damaged a drive may be. If it fails to boot at all, think corrupt boot sector. If the operating system starts to load, then hangs or loops (restarts infinitely), there may be corrupted drivers, corrupt DLLs, Registry problems or a device conflict.

If you haven't installed any new hardware recently and all your internal cards and cables are seated firmly, suspect software error.

The first thing to do is retrieve as much important data as possible from the drive. That said, it always pays to install a new hard drive if drive corruption has been the problem.

If your hard drive crashed hard enough to corrupt data, chances are it crashed hard enough to damage the disk. Even if you're able to wipe the old drive and reinstall Windows successfully, a little scratch might continue to cause problems, leading to multiple failures in the future.

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First published in PC Plus Issue 305

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