LG OLED C1/C2 4K 120 Hz HDR No Signal Fix

Quick Answer

When an LG C1, C2, G1 or G2 works at 4K60 but shows No Signal, black screens or repeated reconnects at 4K120 with HDR, simplify the HDMI chain before changing picture quality controls. Connect the console or PC directly to the television with a certified Ultra High Speed HDMI cable, enable HDMI Deep Color for that input, establish a stable 4K60 SDR baseline, and add 120 Hz, HDR and VRR one at a time.

This sequence separates five different failures that look identical from the sofa: an input not configured for the enhanced format, a marginal cable, an AV receiver or switch that cannot pass the selected combination, an unsupported GPU/console output, or stale HDMI capability negotiation. Black Level, OLED Pixel Brightness and white balance cannot repair any of them.

No-Signal Symptoms That Matter

Record the exact failure instead of calling every interruption “flicker”:

The LG No Signal screen appears immediately after selecting 120 Hz.

The picture returns at 60 Hz but disappears when HDR or 10-bit output is added.

The display repeatedly goes black and Windows plays device-connect sounds.

Green sparkles, magenta flashes or horizontal corruption appear before the link resets.

Xbox 4K TV Details marks 120 Hz, HDR or Dolby Vision as unavailable.

PS5 allows 4K but falls back when 120 Hz output starts.

A receiver passes 4K60 HDR yet fails only with a high-frame-rate game.

Near-black areas pulse while the picture remains connected. That last symptom is usually VRR gamma fluctuation and belongs to a different diagnosis.

Note whether the failure occurs during an intentional mode switch. A single short black interval can be normal while HDMI resynchronizes. Remaining black, looping every few seconds, or losing the device until reboot is not.

Likely Causes in Bandwidth Order

The intermediary is the first suspect when direct source-to-TV operation works. Every device in the path must support the selected resolution, refresh rate, HDR format, chroma/bit depth and optional VRR simultaneously. A soundbar's eARC support does not prove that its HDMI inputs pass 4K120.

The cable becomes more likely when higher data-rate formats trigger sparkles, colored flashes or reconnects. “8K” on a product page is marketing; the HDMI Forum's Ultra High Speed certification and QR label are stronger evidence. Very long passive runs have less margin.

HDMI Deep Color or input configuration matters because the LG must accept the enhanced signal on the selected port. The 2021 C1 manual documents 4K at 100/120 Hz with Deep Color enabled on C1 inputs. Menu wording can change by firmware and region.

Source output can be internally inconsistent. A PC may request a chroma, bit-depth and refresh combination its GPU or adapter cannot provide. A console may retain stale display capabilities after moving through a receiver. Driver updates and standby can also trigger a fresh, unsuccessful handshake.

Port or hardware failure is possible but should be last. It becomes credible when one input fails with multiple known-good sources and cables while another input succeeds under the same format.

Step-by-Step 4K120 Fix

1. Map the physical path

Write down source, cable, switch, receiver or soundbar, and the LG port number. Identify the port labeled eARC separately; eARC affects return audio, not whether every receiver input can transport 4K120 video. Disconnect unused HDMI devices for the first test because CEC activity can complicate restarts.

2. Build a direct low-bandwidth baseline

Connect PS5, Xbox or PC directly to the TV. Select 3840×2160 at 60 Hz with HDR and VRR off. Confirm a stable picture for several minutes. If 4K60 already fails, inspect cable seating, try another TV input and use LG's no-picture troubleshooting before pursuing a 120 Hz-specific theory.

3. Verify the LG input

Enable HDMI Deep Color/4K for the active input using the menu shown by your firmware. Restart the source after changing it so capabilities are read again. Do not copy a C3/C4 menu path; C1 uses webOS 6-era organization, while C2 uses the 2022 layout.

4. Add 120 Hz without HDR

Request 4K120 SDR. On Windows, use Advanced Display and confirm that the active signal—not merely the desktop mode—shows the expected resolution and refresh. On Xbox, inspect 4K TV Details. On PS5, use Screen and Video information. If this step fails, the issue is not HDR tone mapping.

5. Add HDR, then VRR

Enable HDR and retest. Next enable VRR/G-SYNC/FreeSync. Adding features separately shows which negotiation crosses the reliability boundary. For a PC, temporarily select a standard RGB/YCbCr format and supported bit depth rather than forcing the largest number. Once stable, improve the format within the GPU and TV's documented capabilities.

6. Replace only the evidence-backed component

If direct 4K120 HDR fails with link errors, try a short certified Ultra High Speed cable. Test its certification label rather than price. If the direct path works, reconnect the AVR or switch and consult its exact model documentation for 4K120, VRR and HDR pass-through. Firmware-update that device before assuming the television is incompatible.

7. Clear stale negotiation

Shut down the source, TV and intermediary normally. Disconnect power briefly where each manufacturer permits, reconnect the intended topology, power the display and audio device first, and start the source last. This can clear stale EDID/HDCP state, but a restart that only helps temporarily signals an unresolved compatibility problem.

C1/G1 Versus C2/G2

C1 and G1 are 2021 models. Their commonly discussed 40 Gbps HDMI input bandwidth does not mean they cannot display documented 4K120 HDR formats. It means the source must use a supported combination rather than an unnecessary 48 Gbps assumption. The panel workflow is 10-bit; forcing a 12-bit headline value offers no automatic image benefit.

C2 and G2 are 2022 models. Specialist testing reports full-bandwidth HDMI inputs on C2, but that does not improve a weak cable or an older receiver. The 42-inch C2 supports the family's gaming use despite its smaller size; brightness differences are unrelated to HDMI transport.

All four models store settings per input and signal. HDMI link stability must be established before evaluating HGiG, Dynamic Tone Mapping or calibration. A perfect 4K120 handshake can still have incorrect HDR settings, but those are separate stages.

When Not to Change Picture Settings

Do not raise Brightness, alter black level, copy white balance, disable OLED protection or enter the service menu. A missing digital link carries no picture data for those controls to improve. Likewise, repeatedly factory-resetting the TV erases useful state without proving whether the cable or receiver works.

Do not replace every cable at once. Keep one known configuration, change one component, and label the result. Avoid unofficial firmware downgrade packages. If a public update appears related, record versions and reproduce the failure before and after the supported update.

Escalate to LG when a specific HDMI input fails at basic formats with multiple direct known-good sources and certified cables, when internal menus show corruption, or when the TV power-cycles. Escalate to the receiver or source vendor when the direct TV path is stable but their device reintroduces the failure.