Rules
Part of How to choose and set up a monitor or television, new or used
Monitor or TV: no signal, blank screen, blur or wrong colour
A symptom-led order for diagnosing a blank, blurry, flickering or wrong-colour monitor or TV, plus Freeview aerial checks and when to stop and claim under UK law.
What to take away
- Run the screen's own picture test first. Samsung and LG both build one into their TVs. A clean test image suggests the fault is in the source, cable or signal. A faulty one makes the display itself suspect.
- Strip the chain to one source, one cable and one input. Microsoft's own first steps for a dead external monitor are to remove docks, dongles and adapters, then swap the cable and the port.
- A cable that passes a low mode can fail a higher one. HDMI.org describes High Speed cables as up to 10.2 Gbps (4K at 30 Hz) and Premium High Speed cables as up to 18 Gbps (4K at 60 Hz with HDR).
- For HDR colour fringing over HDMI, Microsoft suggests dropping to 30 Hz or to 1920 by 1080 at 60 Hz, or using DisplayPort.
- Missing aerial channels are a separate fault. Freeview's order is: Channel Checker, retune, check equipment, check interference, then the aerial.
Stop first if it is unsafe
Editorial suggestion: unplug the display and stop if there is smoke, a burning smell, sparks, liquid inside, a damaged mains lead or a screen that is unstable on its stand. Do not open a television or monitor, even when unplugged.
Under the Great Britain ecodesign rules for electronic displays, makers may require professional repairers to show technical competence, compliance with the Electricity at Work Regulations 1989 and liability insurance before they hand over repair information (Schedule 19). Check also whether your model is on the GOV.UK product recalls and alerts list (GOV.UK). The repair diagnosis guide explains when to stop DIY checks.
Blank screen or "no signal"
Work down the list and stop when the picture returns. Steps 1 to 4 and 6 are Editorial suggestions; step 5 follows Microsoft.
- Power.Check the power light and the socket. Try another lead if it is detachable.
- Input.Select the correct input by hand rather than relying on auto-detect.
- Direct baseline.Switch everything off. Remove docks, splitters, switches, receivers and adapters. Connect one source straight to one input with one cable you trust.
- Swap one thing at a time.Try another cable, then another port on the display, then another source. If you have a second display, test the source on it. That gives three separate questions: is the cable, the source or the display at fault?
- On a Windows PC,Microsoft says to press the Windows logo key and P and check that Extend is selected, remove docks and adapters, swap the cable, try another video output, and press Windows, Ctrl, Shift and B to reset the graphics driver. If that fails, restart, then roll back the display driver in Device Manager (Microsoft).
- Lower the demand.Pick a basic mode, such as 1080p at 60 Hz, and add features back one by one.
The cable category matters when a mode fails. HDMI.org says High Speed cables reach 10.2 Gbps and 4K at 30 Hz, Premium High Speed cables reach 18 Gbps and support 4K60 with HDR, and Ultra High Speed cables reach 48 Gbps with 8K at 60 Hz and 4K at 120 Hz (HDMI.org). Editorial suggestion: an old cable can show a picture at 1080p yet go blank at 4K60, so buy the category you need, and see the cable table in the monitor and television buying guide.
Is it the display or the source?
Use the display's own test. A built-in test image does not rely on any cable.
- Samsung:on TVs from after 2015, go to Settings, Support, Device Care, Self Diagnosis, then Picture test. Samsung says to include the test result if you contact it for service (Samsung UK).
- LG:on webOS TVs bought in 2021 or later, go to All Settings, Support, Screen Self-Diagnosis. For 2020 sets the route is Support, Additional Settings, Picture Test. LG says the test checks only the TV, not the broadcast signal, so a pass points to the signal (LG UK).
- Other brands and monitors:look in the on-screen menu for a self-test or built-in pattern, and check the manual. Not every model has one.
Reading the result (Editorial suggestion):
Built-in test / Likely area
- Clean image
- Source, cable, port or mode
- Same line, patch or flicker
- The display panel or its electronics
- Test not available
- Use a second source and a second cable instead
Blur, soft text and cut-off edges
Blur has more than one cause, so name it first.
- Soft text on a computer:Editorial suggestion: open display settings and confirm the screen is at its native resolution, then check the scaling setting, because a high-resolution screen at 100 per cent scaling can make text too small and a wrong scale can look soft.
- Edges missing or picture zoomed:this is usually an aspect-ratio or overscan setting. LG's advice is to pick a non-zoom aspect ratio and, for a single device, to open that device's display settings and switch off zoom, overscan or screen-fit options (LG UK). Set-top boxes, streaming devices and consoles may control picture size themselves.
- Smearing in motion:this is separate from resolution. Editorial suggestion: look for a response-time or motion setting in the menu and compare with it off.
Flicker, dropouts and wrong colour
Flicker or blanking at a high mode. Lower the refresh rate. In Windows 11, go to Settings, System, Display, Advanced display and use "Choose a refresh rate". An asterisk marks a rate that does not suit the current resolution, and choosing it changes the resolution (Microsoft). If the fault disappears at a lower rate, suspect the cable, adapter or port at the higher one.
Washed-out or dark HDR. Microsoft says the PC and display must both support HDR, and that HDR over DisplayPort is preferred where the display offers it. If colour fringing appears over HDMI, lower the refresh rate to 30 Hz or the resolution to 1920 by 1080 at 60 Hz. It also says Windows may default to duplicating an external monitor, which disables HDR, and that the "SDR content brightness" slider balances normal content against HDR. Night light can make colours look red or oversaturated (Microsoft).
Dark picture from a PC on a Samsung TV. Samsung says this typically happens because the TV treats SDR content from the PC as HDR. Its fixes are to adjust the Windows SDR content slider, turn on Input Signal Plus (called HDMI UHD Colour on 2018 and older models) for the PC's port, and raise TV brightness (Samsung). Editorial suggestion: other brands name this setting differently, so check the manual.
Grey blacks or crushed shadows. Editorial suggestion: look for a range or black-level setting on both the source and the display. If one end sends one range and the other expects another, the picture looks flat or crushed. Make the two match, and change one end at a time.
Editorial suggestion: do not use a factory reset as a first step. It removes settings and accounts but cannot repair a bad panel, backlight or port.
Aerial and Freeview problems
If apps and HDMI inputs work but broadcast channels fail, treat it as a reception fault. Picture settings do not affect it. Freeview's own guide sets this order (Freeview):
- Channel Checker.Enter your postcode and house name or number to see planned maintenance, transmitter faults and live issues. If it reports a fault or engineering work, that is the likely cause.
- Retune.Accept the prompt, or run an automatic search through first-time installation. If that fails, tune manually with the frequencies from the checker.
- Check equipment and cables.Look for worn leads and bent pins, and secure power and aerial connections.
- Check for mobile interference.Freeview says mobile broadband can interfere with TV signals and that a free filter can be fitted between the aerial cable and the TV.
- Check the aerial.Remove unnecessary amplifiers and splitters, check the external aerial for weather damage or movement, and ask a landlord or management company about a shared aerial.
- Call an installer.Freeview recommends a qualified installer and getting more than one quote.
Freeview adds that appliances such as fridges and tumble dryers, and weather such as morning fog, can disrupt reception (Freeview). The Channel Checker is a prediction for the area around an address, not a guarantee.
When to stop and seek service
Editorial suggestion: seek service when the test image shows the fault, when the problem appears on every input and menu, when a port is loose, or when you cannot get a stable baseline. Give the repairer the model, serial number, the cables and modes you tried, photos, and the picture-test result. Ask for a written quote, as described in repair quote problems.
If you bought from a UK trader, the Consumer Rights Act 2015 applies across the United Kingdom:
- A short-term right to reject faulty goods normally lasts 30 days from delivery (section 22).
- A fault within six months of delivery is presumed to have been present at delivery for repair and replacement claims (section 19).
- Repair or replacement must be done within a reasonable time and without significant inconvenience to you (section 23).
Editorial suggestion: a fault that follows one source or one cable is not a case for a workshop. The setup baseline is covered in the monitor and television buying guide.
Sources
- Run a picture test - Samsung UK
- Troubleshooting picture and sound with Self Diagnosis - LG UK
- Reception, picture and channels troubleshooting guide - Freeview
