Quick answer
Unplug the soundbar and the subwoofer from the wall, wait a full 30 seconds, then plug the sub in first and the bar second. Most subs re-link on their own inside a minute. If the green LED on the back of the sub keeps blinking, run the manual pairing below. Solid green means you're connected.
Read the light before you touch anything
I've watched plenty of people box up a working subwoofer because nobody told them what the LED means. On the back of an LG wireless sub, solid green is paired and working. Blinking green means it's searching for the bar and hasn't found it yet. Red is standby or a failed link, and a red light while the soundbar is off is completely normal.
One more thing before you start unplugging. If you've got a solid green light but no bass, pairing isn't your problem. Grab the remote, find the Woofer Level control (WF or W.LVL on most LG remotes), and check it isn't parked at the bottom of its range. A sub turned all the way down behaves exactly like a sub that isn't there.
The clean power cycle
Half the "dead" subs I've dealt with come back with nothing fancier than this, done in the right order.
- Unplug both the soundbar and the subwoofer from the outlet. Not standby. Out of the wall.
- Wait 30 seconds minimum. The internals need time to fully discharge or you haven't reset anything.
- Plug the subwoofer back in first and give it about ten seconds.
- Plug in the soundbar and power it on. Watch the sub's LED. It should go from blinking to solid green within a minute.
Manual pairing, step by step
If the auto link won't take, force it. This is LG's own procedure and it works across nearly every model from the older SL bars to the current S-series.
- Turn the soundbar off, but leave it plugged in.
- Find the PAIRING button on the back of the subwoofer. On some models it's recessed, so keep a pen or paperclip handy.
- Press and hold it until the LED starts blinking green quickly.
- Turn the soundbar on and wait. When the link takes, the LED settles to solid green.
- Still red? Press and hold PAIRING again and run the sequence a second time. It sometimes needs two passes.
When it pairs but keeps dropping
A sub that connects, then cuts out every few minutes, is almost always fighting interference rather than failing. The wireless link between bar and sub lives in the same crowded airspace as your Wi-Fi, and the fix is physical, not buried in a menu. Keep the sub within about 30 feet of the bar with a reasonably clear path between them. Don't park it inside a cabinet or directly behind the TV stand. If your router sits next to the soundbar, move one of them at least three feet. Microwaves and baby monitors are repeat offenders too. If the bass drops out every time someone reheats coffee, you've found your culprit.
Update the firmware while you're in there
LG pushes soundbar fixes through the LG Soundbar app on iOS and Android, and pairing stability on several recent models improved through firmware. Connect the bar to the app and take whatever update it offers. It costs you five minutes and rules out a whole category of gremlins.
When it's the hardware, not you
If you've run the manual pairing twice, cleared out the interference suspects, and the LED still won't hold green, the sub's wireless module or amp board is likely done. Two last checks first. Try the sub on a different outlet on a different wall, because a tired power strip can brown it out. And listen for a soft thump from the driver at power-on, which tells you the amp is at least waking up. LG's warranty runs a year on soundbars; inside that window, use it and stop tinkering. Outside it, repair quotes tend to land near half the cost of a new bar-and-sub set, which is why most people don't bother.
Genuine hardware failure is the rare case here. Work the list in order and the odds are strongly on your side.