Display problems, from the panel to the cable

Why this matters

Display faults are the easiest class in this domain to diagnose properly and the one where guessing is most common, because the symptom is so visible that people jump straight to a conclusion. A blank screen produces "the monitor is dead" far more often than it produces a sequence of tests.

The reason to do it properly is that the display chain has four links — source, cable, display, and the settings — and substitution isolates each one in seconds. There is almost no other fault in this exam where the diagnosis is as cheap.

The exam asks these as scenarios where the symptom is described precisely. Precision in the symptom is the gift: "no signal message" and "completely dark" are different faults, and reading the difference is most of the answer.

The lesson

No image at all: the order that separates the source, the cable and the panel

Start by reading what the display itself is saying, because it separates two whole categories:

• Completely dark, no backlight, no menu. The display is not powered, or it has failed. Check its power, its switch, and whether its own on-screen menu appears — a menu proves the panel and backlight work.
• A "no signal" or "check cable" message. The display is alive and receiving nothing. The fault is the source or the cable.

Then, for a live display with no signal:

1. Is the input correct? Most displays have several, and they do not always switch automatically. This is the single most common cause.
2. Is the cable in the right port on the machine? The motherboard port on a machine with a graphics card, from lesson 18, is the classic error.
3. Reseat both ends.
4. Swap the cable.
5. Try a different port and cable type, which also tests a failed output.
6. Try a different display, or the same display on a different machine. These two tests, in either direction, isolate the fault definitively.
7. Is the machine actually running? Lesson 35: no image with no POST is a different problem. Look for drive activity, listen for beeps, check whether a keyboard shortcut that would shut down the machine works.

Laptops add one step: the display toggle. A laptop set to output only to an external display shows nothing on its own panel, and the fix is a function key. Confirm by attaching an external display and looking for the desktop there.

Wrong image: resolution, refresh rate, scaling and colour depth

An image that is present but wrong is a settings problem far more often than a hardware one.

• Blurry text or a stretched picture. The display is not running at its native resolution. Flat panels have exactly one native resolution and everything else is interpolated.
• Everything too large or too small. Scaling, which is separate from resolution on modern systems. A high-resolution display at 100 per cent scaling produces unreadably small text, and that is the setting rather than the resolution.
• Black bars, or an image cut off at the edges. An aspect ratio mismatch, or the display's overscan setting — common when a television is used as a monitor.
• Flicker. Often a refresh rate the panel does not handle well, and historically a major issue with cathode ray displays below 75 Hz.
• Wrong colours, or a missing colour channel. Colour depth setting, a damaged cable, or a bent pin on an analogue connector. A picture with a strong colour cast over an analogue link is almost always a pin or a cable.
• A mode that produces no picture at all. A resolution or refresh rate the display cannot accept. Recovery is booting into a low-resolution mode and resetting it.

Lesson 22's bandwidth budget explains several of these: a display that offers a lower refresh rate at a higher resolution has not failed, it has run out of link capacity.

Physical panel faults: dead pixels, backlight bleed, flicker and burn-in

Faults in the panel itself, and how to recognise each:

• Dead pixels — permanently black. Stuck pixels — permanently one colour. Both are physical, both are permanent, and manufacturer warranties typically specify a minimum number before replacement.
• Backlight bleed and clouding — uneven brightness near the edges, most visible on a dark image. Common to some degree on edge-lit panels, and a fault only when severe.
• Flicker at low brightness, which on some panels is the dimming method rather than a failure, and which some people are sensitive to.
• Dimming, or a picture visible only under a torch. The backlight has failed, from lesson 3, and the torch test is the definitive check.
• Burn-in and image persistence. OLED panels can retain a static image permanently. Persistence on an LCD is usually temporary and clears.
• Lines and blocks. A vertical or horizontal line across every image, or a block of corrupted pixels, is a failed panel or a failed connection to it — and on a laptop, worth distinguishing from a video cable fault by seeing whether an external display is clean.
• Physical damage. Cracks and impact marks; pressure damage shows as a dark blotch with radiating lines.

The single most useful test for all of these: does the fault appear on an external display? If yes, it is the source. If no, it is the panel or its cable. That one question splits the entire category.

Projector and external display faults, including lamp and overheating behaviour

Projectors have their own failure set and the exam includes them:

• Dim image. The lamp ages and loses output over hundreds of hours. Most projectors report lamp hours, and a dim image with high hours is a lamp approaching end of life rather than a fault.
• Sudden shutdown, fans continuing. Overheating. The most common cause is a blocked intake filter, and projectors need theirs cleaned on a schedule. Note that the fans run on after shutdown deliberately, to cool the lamp, and cutting power during that period shortens lamp life.
• Colour wheel noise or a flickering colour cast on single-chip designs.
• Keystone distortion — a trapezoid image, corrected by the projector's own keystone setting or by repositioning.
• Out of focus, or unable to focus, which may be a lens setting or a throw distance outside the projector's range.
• Image with no signal source visible, which is the same input-selection question as any display.

Handling points: lamps get very hot and must cool before replacement, some contain mercury and require correct disposal, and touching a bare lamp with bare fingers shortens its life because of the residue.

Multiple display faults that are really configuration rather than hardware

When one display in a multi-monitor setup misbehaves, the cause is usually configuration:

• A display detected but showing nothing. It may be set to "extend" with nothing on it, or positioned so that nothing ever moves there. Identifying the displays in the operating system's settings confirms which is which.
• Windows opening on a display that is switched off, which is the same arrangement seen from the user's side.
• The third display not working at all, which is lesson 22's graphics limit rather than a fault.
• Displays in the wrong physical order, so the pointer leaves one side and appears on the wrong side of another. Configuration, not hardware.
• Different scaling on different displays, producing windows that change size as they are dragged across.
• A display that works alone and not alongside another, which points at bandwidth on a shared link or a dock's limit.

The test that separates configuration from hardware: connect the failing display on its own, directly to the machine. If it works alone, nothing is broken and the answer is in the settings, the dock, or the graphics limit. That is the last of this lesson's substitution tests, and like the others it costs a minute and settles the question.