The auto-calibration says my subwoofer isn't detected (or the sub went silent after calibration). What's wrong?

Settings, not hardware, nine times out of ten. Do not carry the sub anywhere yet.

Before calibration, set the sub up so the AVR can hear it. Volume knob at halfway, so the test tone is loud enough to detect but not so loud the mic overloads. Crossover dial on the sub at maximum, or the LFE position if it has one; the AVR will manage the crossover, and a sub dial set at 60 Hz fights it. Auto-standby off, because a sub that has gone to sleep will miss the first test sweep and be marked as absent. Cable from the AVR's subwoofer pre-out to the sub's LFE or line input, seated firmly at both ends. Then run calibration.

Sub went silent after calibration? Go into the speaker configuration menu. Auto-calibration sometimes decides "Subwoofer: No", or sets the sub level right down, because it heard too little on the sweep (usually the auto-standby or the low volume knob above). Set Subwoofer to Yes, bring the level back toward zero, and re-run. Also check the front speakers were not set to Large with the sub mode on "LFE only"; in that setting the sub only plays the dedicated LFE track and stays silent on music.

Plays the test tone but silent in movies? That is crossover settings, not the sub. If every speaker is Large, nothing below the crossover is being sent to it. Set the speakers to Small, 80 Hz as a starting point, and the sub wakes up.

Test tone silent too, with all of the above correct? Swap the cable, try the sub on a different input, and check the sub's own power light. Only then are we talking hardware.

Five minutes of settings beats a service call. WhatsApp us the AVR and sub models and a photo of the speaker settings screen, and we can usually spot which line is wrong.