Outdoor Unit Serial Signal (Communication) Error

What it means

This code appears on Toshiba residential split air conditioners when the fault code is read with the remote check function. It means the indoor unit is not receiving the normal serial communication signal from the outdoor unit over the connecting cable. The outdoor unit stops so there is no cooling, while the indoor fan may keep running. The same code can mean something else on other brands, and Toshiba commercial units use different codes.

Models covered: Toshiba residential split air conditioners (RAC) that show a two-character check code through the remote check function; Toshiba commercial units use different codes.

Related codes: 121714

Symptoms4

• The indoor fan runs, the air is not cold and the outdoor unit is stopped.
• The indoor unit lamps blink and code 04 is found with the remote check function.
• The fault appears after a power cut or with voltage swings.
• The unit runs and then stops after a few minutes, over and over.

Possible causes5

• No power to the outdoor unit, or a blown fuse on its board.
• Loose or wrongly ordered terminals on the interconnecting cable.
• A cable damaged by sun, heat, rodents or water in the joints.
• A faulty outdoor control board, a common cause of this code.
• A fault in the communication circuit on the indoor board.

How to diagnose6

1. Read the code with the remote check function: press and hold the check button, then step through the codes with the timer button until the indoor unit beeps continuously; the code shown then is the fault. Steps vary slightly by remote, so follow the owner's manual.
2. Switch off at the breaker for several minutes, restart and see whether the code returns.
3. Switch off at the breaker before opening any panel; the outdoor unit and its inverter board keep a dangerous charge for a while after switch-off.
4. The technician checks the interconnecting cable terminals at both units for order, water and corrosion.
5. The technician measures the supply reaching the outdoor unit and checks its board fuse and diagnostic lamps.
6. If supply and wiring are sound, both boards are tested to find the faulty side.

How to fix6

1. Retighten the interconnecting cable terminals and correct their order.
2. Replace the fuse after finding why it blew, and restore supply to the outdoor unit.
3. Replace a damaged interconnecting cable.
4. Fit a voltage protector if the supply fluctuates.
5. Replace the faulty outdoor or indoor control board.
6. Run the unit and confirm the outdoor unit starts and the code does not return.

Repair cost

In Saudi riyals; the highlighted part is the expected range for this fault.

The cost is low for cable or terminal repairs, and rises sharply if the outdoor unit needs a new inverter board.

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