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Diagnosing Monitor Problems

1. Troubleshooting monitors

Generally speaking, there isn't a huge amount of troubleshooting that needs or can be done with modern screens. With the progression from CRT (Cathode Ray Tube) to LCD (Liquid Crystal Display), reliability has increased and many common issues suffered with older displays such as flickering or interference from magnetic sources are simply not seen any more. 

There are, however, some basic things that should be checked before resorting to repairing or replacing the monitor. 

1.1 Cables 

Are the correct cables for the device being used and are they plugged in correctly?

At present there are several standards for transmitting video via cable, the two most common when dealing with almost all desktop computers are VGA and DVI (although more modern standards such as HDMI and DisplayPort are gradually replacing these.). 


VGA video is generally carried by a 15 pin D-SUB connector type cable often characterised by blue connectors at either end (shown below). 

D-SUB connector type cable


There are several flavours of DVI video but in general you will find a DVI-D Single-link cable often featuring a white connector (shown below).


 DVI-D Single-link cable


You should ensure that both ends cables are pushed securely into the device and the fastening screws (if present) are reasonably tightened. The connector should not wobble when pressure is applied.  

1.2 Defective pixels 

Sometimes when you buy a new monitor or after some use you might notice one or more of the pixels in the display doesn't respond to the video signal in the same way as the others. The ISO standard for pixel faults distinguishes between three types of defective pixels, 'hot', 'dead' and 'stuck'.

- A 'hot' pixel is one which is always 'on', characterised by an unchanging, white dot. 

- A 'dead' pixel is the opposite, stuck in the 'off' position. This is characterised by an unchanging black dot.

- A 'stuck' pixel occurs when one or more sub-pixels (red, blue or green) are always on or always off. This could manifest as an unchanging dot of any colour. 

A defective pixel generally cannot be fixed by the user, if the monitor/machine is under 4 weeks old then it can usually be returned or replaced under the 'Sales of Goods Act 1979'. For slightly older equipment a repair or replacement might be available under the warranty offered by the manufacturer.   

1.3 Laptops

If you are using a laptop and the screen has failed, a quick test to identify if it's the screen or video card is to connect it to an external monitor. Depending on the age, make and model of your laptop will depend on the type of video connections it will feature. Older Windows laptops will generally all have a VGA output and newer machines tend to include a HDMI connection. Apple laptops vary again but almost all the new machines require a converter in order to connect a standard video cable. Again these vary depending on model but these will generally be in the form of a DVI-D or HDMI connection. 

Once you've established which cable is required to connect the laptop to the external monitor you should do so, making sure both ends of the cable are connected securely. If the machine transmits a signal to the screen then it would indicate that the problem is more than likely to do with the onboard screen. If there is no signal being passed to the monitor, however, then this would suggest that the issue might lie with the machines video adapter.

2. Video Adapter Problems

Before assuming the monitor to be faulty, it is worthwhile substituting it with another in order to establish that the video card of the PC is working. Faults such as on-screen artefacts (shown below) or freezing/stability issues that are still present when using a different display from normal are indicative that there might be a problem with your video adapter. 

on-screen artefacts


On a desktop machine, connections to the video adapter are generally found on the rear panel. There are often several types of connection which differ with the age, make and model of the machine. The most common types are shown below. 

video adapter connections

Many modern desktops and laptops feature an integrated video adaptor built into the motherboard. This can be problematic if a fault develops as the whole mainboard may need to be replaced. As mentioned in the previous section, if the machine is under four weeks old then a repair or replacement should be covered by the 'Sales of Goods Act 1979' free of charge, if the machine is older a repair or replacement might be covered by the manufacturer’s warranty.

Due to the several different interface ports in current use, it's worth checking that the interface is compatible with the monitor. In the case of dual head interfaces, where there are two or more output ports, the signal could be directed to the wrong connector. The interface driver software can correct this. Where the monitor has several ports, it may be expecting signals from an unused connector. The monitors setup will redirect the signal appropriately. If the driver software has not been installed, the interface is likely to operate only in its basic mode. Installing the software, together with any updates, will make the full range of modes of the interface available.


Written by IT Services. Latest revision 2 September 2019