Monitor Says No Signal? Input Sources, HDMI vs DisplayPort, and Missing HDMI Audio

When a powered-on monitor shows “No Signal,” the message means exactly what it says: the monitor is awake but nothing usable is arriving on the input it’s currently watching. Nine times out of ten the cure is embarrassingly simple — the monitor is set to the wrong input source, so pressing the input/source button to select the port the cable is actually plugged into is the correct first move. Only after the input selection is confirmed should you move on to cables, the computer’s output settings, and the video card itself.

Check the input source before anything else

Modern monitors carry several inputs — commonly one or more HDMI (High-Definition Multimedia Interface) ports plus a DisplayPort, and sometimes legacy VGA (Video Graphics Array) or DVI (Digital Visual Interface). The monitor displays whichever single input it’s told to watch. Some monitors auto-detect an active signal and switch to it; many don’t, or auto-detect unreliably, especially after a new device is connected or after waking from standby.

So the first step costs nothing and takes five seconds: press the monitor’s Input or Source button (or find the input list in its on-screen menu) and cycle to the port your cable occupies. This is the classic “check the obvious” step of methodical troubleshooting — before opening Device Manager, before swapping hardware, verify the two ends of the conversation are even on the same channel. A laptop happily extending its desktop to a monitor that’s listening on the wrong port produces precisely the powered-monitor, active-laptop, “No Signal” combination that fools people into replacing cables.

The swapped-computer scenario: same monitor, different port

Here’s a pattern that trips up real deployments and exam questions alike. A monitor was used with an old computer through its DisplayPort input. The computer is replaced; the new machine gets connected with a known-good cable — but into the monitor’s HDMI port, or from a different output on the new PC. The monitor is still set to the DisplayPort input it used for years, so it reports “No Signal” even though everything is healthy. The most likely explanation isn’t a dead PC or a bad cable — it’s that the monitor is watching an input no longer in use. Select the input that matches the new connection and the picture appears.

The lesson generalizes: whenever the hardware arrangement changes — new PC, new cable, new docking station — re-verify the input selection before diagnosing the new equipment. A “known-good cable” proves the cable, not the input routing.

HDMI vs DisplayPort: what a technician should know

Both are digital standards that carry video and audio over one cable, which is why either can drive a monitor’s built-in speakers. Their practical differences matter when you’re choosing a port, explaining behavior, or answering exam questions.

Attribute	HDMI	DisplayPort
Primary ecosystem	TVs, consumer electronics, most monitors	Computer monitors, docks, professional graphics
Audio carried	Yes, embedded with video	Yes, embedded with video
Connector retention	Friction fit only	Latching connector on full-size plugs
Multi-monitor from one port	No native daisy-chaining	MST (Multi-Stream Transport) daisy-chaining
Common variants	Full-size, Mini, Micro	Full-size, Mini DisplayPort
Typical use call	Connecting to TVs and home theater gear	High-refresh and multi-display desktop setups

For a single office monitor, either port works; the failure mode to internalize is that they are separate inputs on the monitor, each selectable independently — a signal on one does nothing while the monitor watches the other.

Video works but the monitor’s speakers are silent

A related service call: video over HDMI is perfect, but no sound comes from the monitor’s built-in speakers even though audio plays fine through the laptop’s headphone jack. Because HDMI carries audio as a separate playback device, the thing to check first is the operating system’s default audio output device. When an HDMI display is connected, the OS registers it as a new sound output (it appears by the monitor or GPU name in the sound settings), but the OS doesn’t always switch to it — audio keeps routing to the laptop’s internal speakers or jack.

The fix: open the sound output settings (Windows: Settings > System > Sound, or the speaker icon’s output picker; macOS: System Settings > Sound > Output) and select the HDMI/monitor device as the default output. Only if the correct device is selected and silent do you go further — monitor speaker volume/mute in the on-screen menu, a cable that’s HDMI end-to-end (a DisplayPort-to-HDMI adapter chain can drop audio depending on direction and adapter), or a muted application mixer. Checking the OS output device first beats swapping cables because it’s the most common cause, it’s software-only, and it takes seconds to confirm.

The rest of the no-signal checklist

If the input source is right and there’s still no picture, walk outward methodically: reseat both cable ends (DisplayPort latches can half-seat), swap in a known-good cable, and try a different port on both monitor and computer. On a desktop with a discrete graphics card, make sure the cable is in the card’s output, not the motherboard’s disabled onboard video — a top-tier classic. On laptops, use the display-output toggle (Windows key + P) to confirm the external display is set to duplicate or extend rather than “PC screen only.” Finally, test the monitor on another computer to decide which side of the cable owns the fault. Identifying ports by sight helps here — the guide to computer cable and connector types covers the full lineup. And if the screen shows a picture but it’s unstable rather than absent, that’s a different tree — see fixing a flickering monitor; a picture that’s present but dim points at backlight trouble instead, covered in dim screens and dead pixels.

How the 220-1201 exam tests this

• FIRST-step questions: monitor powered, laptop active, “No Signal” on screen, multiple inputs present — what should be done FIRST? The credited answer is selecting the correct input source on the monitor; distractors include replacing the cable, updating drivers, and rebooting.
• Changed-equipment inference: a new PC replaces an old one that used the monitor’s DisplayPort input, and the monitor now shows “No Signal” through a known-good cable. The MOST likely explanation is that the monitor is still set to the previous input rather than the port now in use.
• HDMI audio routing: picture over HDMI works but the monitor’s speakers are silent; the FIRST check is the OS default playback/output device, because HDMI audio registers as its own output that the OS may not have selected.
• Least-invasive ordering: across all these, the exam rewards the cheapest, most probable, software-or-button-level check before hardware replacement — the same probable-cause discipline the troubleshooting methodology objective demands.

Display troubleshooting is one slice of the Hardware and Network Troubleshooting domain — the full 220-1201 study guide maps every domain and how heavily each is weighted. FIRST-step instinct comes from repetition, and scenario practice questions are the cheapest way to get it.

Quick reference

• “No Signal” = monitor is powered but its currently selected input has no active source; it is not proof of dead hardware.
• First action: press Input/Source and select the port the cable actually uses — especially after any equipment change.
• Old PC on DisplayPort, new PC on HDMI (or vice versa) → monitor still watching the old input is the most likely cause.
• HDMI and DisplayPort both carry video plus audio; they are independent, individually selectable inputs.
• Silent monitor speakers with working video → set the HDMI/monitor device as the OS default audio output first.
• Desktop with a discrete GPU: plug into the card, not the motherboard’s video port.
• Laptop external displays: confirm duplicate/extend with Windows key + P before suspecting hardware.
• Isolate remaining faults with known-good cable swaps and a second computer.