U4
Mitsubishi Electric Split AC
Severity: MinorWhat Does This Error Mean?
U4 on a Mitsubishi Electric AC means the indoor unit cannot receive the signal from the remote controller. This is usually a simple problem — dead batteries in the remote, an obstruction between the remote and the indoor unit sensor, or direct sunlight hitting the indoor unit receiver. In rare cases it indicates a fault with the indoor unit IR receiver board.
Affected Models
- Mitsubishi Electric MSZ-AP Series
- Mitsubishi Electric MSZ-EF Series
- Mitsubishi Electric MSZ-GL Series
- Mitsubishi Electric MSY Series
- Mitsubishi Electric Inverter Split AC (all models)
Common Causes
- Remote control batteries are dead or too weak to transmit
- Object blocking the line of sight between the remote and the indoor unit receiver
- Bright sunlight or another strong infrared source interfering with the receiver
- Remote control IR emitter LED has failed
- Indoor unit IR receiver module faulty or damaged
- Remote control wired version (PAR series) — communication wiring fault
How to Fix It
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Replace the batteries in the remote control.
Use fresh alkaline AA or AAA batteries (depending on your remote model). Even if the remote display is working, weak batteries may not transmit a strong enough IR signal.
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Point the remote directly at the indoor unit receiver (the small lens at the front of the unit) from within 5 metres.
Remove any curtains, shelves, or objects between the remote and the unit. Check that nothing is covering the IR receiver on the indoor unit itself.
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Check for strong light sources near the indoor unit.
Direct sunlight, halogen spotlights, or other bright lights pointed at the indoor unit receiver can overwhelm the IR sensor. Shade the indoor unit receiver or move the light source.
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Test the remote using a smartphone camera.
Point the remote at your phone camera and press any button. Most phone cameras can see infrared light — if the remote is working, you will see a faint purple-white flash from the remote's end. If you see no flash, the remote IR emitter has failed and the remote needs replacing.
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If the remote is working but U4 persists, call a technician.
The indoor unit IR receiver module may need replacement.
Frequently Asked Questions
Where is the IR receiver on a Mitsubishi Electric indoor unit?
The IR receiver is the small dark plastic lens usually located at the right side of the front panel of the indoor unit. It is the same area where the indicator light is. Point the remote directly at this spot from a clear line of sight.
Can I control my Mitsubishi Electric AC without the remote?
Yes — all Mitsubishi Electric indoor units have an emergency operation button, usually hidden under the front panel or the service cover. This button runs the unit in a preset mode (usually auto at 23°C). For full control without the original remote, a universal remote or the Mitsubishi Electric MELCloud Wi-Fi adapter can be used.