If your vehicle is showing high beams won't turn on or won't turn off, here are the most likely causes and the parts you'll need to fix each one.
The relay that switches power to the high beams can fail mechanically. If the internal contacts weld together, the high beams will stay stuck on; if the magnetic coil fails, they won't turn on at all.
The stalk on your steering column physically controls the high beam toggle. Internal wear, broken plastic detents, or shorted contacts can prevent it from latching the high beams on or turning them off.
If your vehicle uses dual-filament bulbs (like H4 or 9007), the high beam filament can burn out while the low beam continues to work. Drivers often don't notice until both sides have failed, causing the high beams to 'suddenly' not turn on.
A blown fuse will completely cut power to the high beam circuit, preventing them from turning on. This is usually a symptom of a power surge, a failing bulb, or a short circuit in the wiring.
Installing aftermarket lighting without the proper CANBUS decoders or anti-flicker harnesses can confuse the vehicle's computer. This often results in high beams that won't activate or get stuck on due to incorrect voltage feedback.
In modern vehicles, the BCM electronically controls the lighting circuits rather than a direct switch. A software glitch, water intrusion, or internal short in the BCM can cause erratic headlight behavior, including high beams that refuse to turn off.