Infrared is invisible to your eyes but obvious to your phone's camera — test it, don't guess it. And the cheapest parts are often the disease: two dead batteries dressed up as a terminal AC illness, while the sneakiest lock of all is drawn on the screen.

How the signal flies (a light path your eyes can't see)

A remote speaks infrared — press a key and the LED at the top fires a burst of coded light pulses; the receiver window on the indoor unit "sees" them and obeys. The path has just three segments: batteries and contacts (no power in, no light out — 60% of failures live here); the emitter and the light path (a grimy window, sunlight drowning the signal, metal in between); encoding and locks (child lock holds the signal hostage — the light still blinks, but nothing is on board). So step one is never disassembly — it's your phone's camera: aim the viewfinder at the emitter, press a key — a purple-white flicker = the LED lives, the trouble is elsewhere; total darkness = the remote itself.

Match the evidence

• Dark screen, no blink, plays dead → Batteries and contacts (60%): fresh AAA alkalines (batteries older than six months retire now; never rechargeables — 1.2 V can't push the LED), poles matched to the markings; white-green crust on the contacts gets the pencil-eraser treatment down to bare metal. Still dead? Deep reset: batteries out → hold power 15 seconds to drain → wait 2 minutes → batteries back.
• Screen lit, light blinks, but the AC ignores every key → Emitter and light path: wipe the top window with an alcohol-damp swab (keep liquid out of the bay); wipe the receiver window on the unit (the dark little window by the indicator); sunlight and LED lamps drown IR — close the curtains and retry; close to under 3 m, nothing metallic in between.
• A little lock icon on screen; keys do nothing but the light blinks → Child lock: hold temperature +/− for 3 seconds until the beep (combos vary by brand — the manual's \u201cfunction lock\u201d page settles it); if the combo fails, pull the batteries to reset — the lock dies with the power. The signal never left home; the AC and the remote are both innocent.

The final dividing line lives on the AC itself: press the emergency switch hidden behind the filter cover — if the unit starts, the board is alive and the trouble is the receiver or the remote (a receiver part is $5–10, but opening the panel and soldering is not DIY); if it doesn't, it's the board — call a tech. As for the remote itself: no flicker on camera = dead LED or board, and a repair past $30 loses to a new remote — universal $8–20 (needs a brand code), OEM $15–40. A water-damaged or dropped remote doesn't get repaired — it gets replaced.