Indoor and Outdoor Unit Transmission Error

What it means

This code appears on Daikin split, ducted and cassette air conditioners and means the indoor unit is not receiving valid signals from the outdoor unit, or the reverse, over the communication wiring between them. Cooling stops because the two units cannot coordinate, and the same code can mean something else on other brands.

Models covered: Daikin split air conditioners and Daikin ducted/cassette (SkyAir) units, which share the same error code system; the code shows on the wired remote or via the wireless remote check function.

Related codes: A1E1U2

Symptoms4

• The indoor fan runs but the outdoor unit does not, and U4 appears.
• The code appears right after installation, relocation or wiring work.
• It may appear after a power cut or a thunderstorm.
• The unit sometimes runs, then loses communication intermittently.

Possible causes6

• Communication wires connected wrongly or in the wrong order after installation or relocation.
• A loose or corroded terminal at the indoor or outdoor terminal block.
• A cut or damaged communication wire, or a poor splice inside the wall.
• No power reaching the outdoor unit from a tripped breaker or damaged connection on models with a separate supply.
• Electrical interference or an unstable supply.
• A faulty communication circuit on the indoor or outdoor board.

How to diagnose5

1. Switch the unit off at the breaker for several minutes, then restart.
2. Check that the outdoor unit breaker, if separate, is on.
3. A technician checks the communication wires are in the correct order per the model's wiring diagram and tightly fixed.
4. The technician checks the wire along its length for breaks or poor splices and measures voltage and signal per the Daikin service manual.
5. If the wiring is good, the technician identifies which board is not sending or receiving.

How to fix5

1. Correct the communication wire order and tighten the terminals.
2. Repair or replace a cut or badly spliced communication wire.
3. Restore power to the outdoor unit or repair its breaker and connection.
4. Replace the indoor or outdoor board whose communication circuit is proven faulty.
5. Run the unit and confirm both units work together and the code does not return.

Most cases are solved by correcting the wiring or repairing the communication wire at low cost; the cost only rises when a board, especially the outdoor one, must be replaced.

Prices are rough estimates including the part and the technician’s fee, and vary with brand, model and part source.
Prices last updated: September 2026

Frequently asked questions