Wireless sub with a soundbar, or wired sub with a receiver
The fixes are completely different, so start here. A wireless subwoofer that came boxed with a soundbar pairs over its own radio link — when it goes silent, the pairing dropped. A wired subwoofer connected to a receiver by a single RCA cable goes silent for boring reasons: a setting, a knob, or the cable. Both are below.
Re-pair it — each brand’s way
Samsung (ID SET): with both units plugged in and the soundbar left in standby, hold the small ID SET button on the back of the sub with a paper clip until its blue LINK LED blinks rapidly. Then hold Mute on the soundbar remote until “ID SET” shows on the bar’s display, and power the bar on while the sub is still blinking. Solid blue = paired.
LG: soundbar off; press the pairing button on the back of the sub (hold 5 seconds if its LED shows red); power the bar back on — solid green = paired. LG’s docs add: unplug both for a full minute first if it refuses, and keep the sub within 15 feet.
Vizio: hold the pairing button on the sub about 5 seconds until it blinks, then hold power on top of the soundbar 5 seconds — the front LEDs flash three times when linked. (Details vary a little by model year.)
The five silent killers, in order
Receiver-driven subs almost never break — they get misconfigured.
The sub’s own controls
Volume knob on the sub turned down, or the sub’s auto-standby put it to sleep and quiet dialogue never wakes it. Set its volume to about halfway and switch auto-standby off (or to high sensitivity) if it has the option.
The right jack, fully seated
The RCA cable belongs in the receiver’s SUB OUT / SW / LFE jack — not a random pre-out — and these cables half-unseat behind cabinets constantly. Push both ends home.
The receiver thinks you have no sub
In the receiver’s speaker setup, Subwoofer must be set to Yes/Use. Rerunning the auto-calibration (YPAO on Yamaha, Audyssey on Denon) with the mic fixes this and sets levels at the same time.
The LFE trap (Denon/Marantz especially)
With Subwoofer Mode = LFE and speakers set “Large,” music can send nothing at all to the sub — Denon’s manual says so outright. Set Subwoofer Mode to LFE + Main for bass in everything, always.
Prove it with the test tone
The receiver’s speaker test tone cycles every channel. If the sub thumps on its tone, the hardware chain is fine and the problem is source content or a mode setting; if it stays silent on its own tone, work back up this list.
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More guides: Soundbar not playing TV audio · Yamaha receiver troubleshooting · Soundbar vs surround, honestly
Quick answers
The sub light blinks forever and never goes solid.
Pairing is failing: kill nearby interference for a minute (Bluetooth speakers, baby monitors), move the sub within a few feet of the bar to pair, unplug both a full minute, then rerun the exact sequence — soundbar in standby is the step everyone skips on Samsung.
The sub is connected but I only feel it on explosions.
That is arguably correct behavior — LFE content is occasional by design. If you want bass in music too, that's the LFE+Main setting on receivers, or the subwoofer level control on a soundbar.
Does the phase/0-180 switch matter?
For loudness at your seat, yes; for silence, no. Flip it while music with steady bass plays and keep whichever position sounds fuller — it is free to try and cannot break anything.
The sub hums even when nothing plays.
A ground loop, usually via the coax/antenna line or a cheap power strip. Try the sub on a different outlet circuit first; a coax ground-loop isolator is the $15 cure if it persists.