If 4K@120Hz or VRR won't engage — it drops to 4K60/1080p, flickers, or black-screens — the HDMI chain is failing to negotiate full bandwidth. Three things cause nearly all of it: the wrong HDMI port, an under-spec cable, or the port's HDMI mode set too low. Work them in order.
Fix it
- Plug into a real HDMI 2.1 (4K120) port. This is the #1 cause. Most TVs put 4K120/VRR on only one or two ports — the rest are HDMI 2.0 and cap at 4K60. Check the labels and move the source to a 2.1 port. (LG's C and G series OLEDs are the exception: on those, every one of the four HDMI inputs runs at 48 Gbps. Reports on the B series disagree, so check the spec for your exact model. Don't extend that to the whole brand — the entry-level A-series is 60Hz with HDMI 2.0 inputs and no VRR at all.)
- Set that port's HDMI mode to full. TVs default ports to a limited mode: Samsung Input Signal Plus, LG HDMI Deep Color, Sony Enhanced format, TCL/Hisense HDMI Mode → Enhanced. Without it, 4K120 isn't offered.
- Use a certified Ultra High Speed (48 Gbps) cable. 4K120 needs the bandwidth; a cable fine at 4K60 will drop to 60Hz or flicker/black-screen at 120Hz. Keep the run short.
- Check the source is set to 120Hz. PS5: Screen and Video > enable 120Hz + VRR (per-title) — there's a longer walkthrough if the PS5 gives you 4K HDR but no 120Hz or VRR. Xbox Series X: 4K TV details should report 4K120. PC: select the 120Hz refresh in display settings. If the console gives no picture at all on that port, work through PS5 HDMI not working fixes first.
If it's VRR specifically
- VRR needs the port in its full mode. It does not need a 48 Gbps cable on its own — VRR runs fine inside HDMI 2.0 bandwidth at 4K60 or 1440p120, and the Xbox Series X does it over both TMDS and FRL links. The cable only becomes the limit when you want VRR and 4K120 together.
- Some TVs disable VRR/120Hz when motion interpolation or certain processing is on. Use Game Mode, which turns those off.
- Confirm the TV actually supports VRR on that input — a few only do it on specific ports.
If it flickers or blacks out at 120Hz
That's a bandwidth fault: the cable or port can't sustain 48 Gbps. Swap to a certified short cable, confirm Deep Color/Enhanced mode is on, and drop to 4K60 to confirm the link is otherwise stable. Black-screen-at-120Hz almost always = cable.
A reality check on "120Hz"
Make sure your TV is native 120Hz, not a 60Hz panel advertising "120 Motion." Budget TVs often use motion-interpolation marketing numbers and cannot accept a true 4K@120Hz signal — no setting fixes that.
But a native 120Hz panel isn't a guarantee either, and this catches people out. Panel refresh rate and input capability are separate specs. Plenty of 2019–2020 sets — the TCL 6-Series and Hisense H9F among them — have genuine 120Hz panels behind HDMI 2.0-only inputs. They'll run 1080p at 120Hz, or interpolate to 120 internally, but they cannot accept 4K@120Hz from a console. Native 120Hz is necessary; it isn't sufficient. Check the input spec, not the panel spec.
FAQ
Why does my TV only do 4K60, not 120Hz? Either you're on a HDMI 2.0 port, the port's mode is limited, or the cable isn't 48 Gbps. Move to a 2.1 port, enable Deep Color/Enhanced mode, and use a certified cable. If you're choosing a new set for this, the best gaming TVs for PS5 and Xbox all expose true 4K120 ports.
VRR won't turn on. It needs the full HDMI mode and a certified cable, and Game Mode on. Some processing settings also disable it — turn them off.
It black-screens at 120Hz but 60Hz is fine. That's the cable can't sustain 48 Gbps. Swap to a certified Ultra High Speed cable, kept short.
My "120Hz" TV won't accept 120Hz. Two different causes. Either it's not a native 120Hz panel at all (budget sets advertise interpolated "120"), or it is one but its HDMI inputs are 2.0 and cap at 4K60 — common on 2019–2020 models. The panel spec and the input spec are separate; check the input.