Fujitsu code 11: serial communication error

Fujitsu display

11

“Serial communication error (Serial reverse transfer error) (Outdoor unit)”

KPAS, KPBS, KNAS, KMAS, KZAS, KTAS, ASLH, LZBS, LZAS, LMAS, LPAS · KPAS/KPBS service p. 03-2; KNAS service p. 03-2; KMAS service p. 03-2; KZAS service p. 03-2; KTAS service p. 03-2; ASLH service p. 03-2; LZBS service p. 03-2; LZAS service p. 03-2; LMAS service p. 03-2; LPAS service p. 03-2
Technician

OPERATION 1 / TIMER 1. The indoor unit gets no serial signal from the outdoor unit for more than 2 minutes after power-on, or for more than 15 seconds during normal operation (KPAS/KPBS Service Manual SR_AS142ES_09, p. 03-24). KNAS prints it without “(Outdoor unit)”. KPAS installation manual: “Serial communication error” (KPAS Installation Manual, p. En-16).

What it means

Code 11 means the indoor and outdoor units have lost their serial communication. In the installation manuals’ lamp tables it is one OPERATION blink and one TIMER blink, repeated, while ECONOMY blinks fast; a wired remote controller shows 11 KPAS Installation Manual, p. En-16; LZBS Installation Manual, p. En-11; RLS3 Installation Manual, p. En-11.

The K-series and L-series service manuals list two faults with these blinks KPAS/KPBS Service Manual SR_AS142ES_09, p. 03-2; LZBS Service Manual SR_AS048EF_02, p. 03-2:

• Serial reverse transfer error. The indoor unit receives no serial signal from the outdoor unit for more than 2 minutes after power-on, or for more than 15 seconds during normal operation KPAS/KPBS Service Manual SR_AS142ES_09, p. 03-24.
• Serial forward transfer error. The outdoor unit cannot properly receive the indoor unit’s serial signal for 10 seconds or more KPAS/KPBS Service Manual SR_AS142ES_09, p. 03-26.

For both, the manual lists a connection failure, an external cause, a failed main PCB, or a failed fan motor: outdoor for the reverse error, indoor for the forward error KPAS/KPBS Service Manual SR_AS142ES_09, pp. 03-24, 03-26. The RLS3 Service Instructions give the same detection times pp. 02-03–02-04.

On KPAS and KPBS units the mobile app table adds a third digit: 11.1 and 11.2 are the reverse transfer error at start-up and during operation, 11.3 and 11.4 the forward transfer error at start-up and during operation KPAS/KPBS Service Manual SR_AS142ES_09, p. 03-8. The KPBS remote’s Service check can show that digit KPBS Operation Manual, p. En-12.

Check these first

Fujitsu’s owner’s manuals give no owner fix for this code. What they cover is reading the code and stopping the unit:

1. Check that it is an error code: OPERATION and TIMER blink while ECONOMY blinks fast KPAS Operation Manual, p. En-13.
2. If your remote has Service check, read the code with it and write it down KPAS Operation Manual, pp. En-13–En-14; KPBS Operation Manual, p. En-12; KNAS Operation Manual, p. En-11; KZAS Operation Manual, p. En-16; KTAS Operation Manual, p. En-15; ASLH Operation Manual, p. En-11; LPAS Operation Manual, p. En-13.
3. Stop the unit and turn off its breaker. The manuals say: “Immediately stop operation and turn off the electrical breaker in following cases. Then consult authorized service personnel.” One of the cases is “The OPERATION indicator and TIMER indicator blink while the ECONOMY indicator is blinking fast.” RLS3 Operation Manual, p. En-10. The K-series and L-series manuals print the same rule with lamp symbols KPAS Operation Manual, p. En-12; LZBS Operation Manual, p. En-13. This is separate from their rule against starting or stopping the unit with the breaker or the plug in everyday use KPAS Operation Manual, p. En-2.
4. Give the code to authorized service personnel when you call KPAS Operation Manual, p. En-14.

When to call a licensed technician

Everything after reading the code is service work in Fujitsu’s manuals. The KPAS/KPBS service manual’s checks for 11 KPAS/KPBS Service Manual SR_AS142ES_09, pp. 03-24–03-27:

• The technician first resets the power and sees whether the error shows again. If it does not, the manual points to external causes: the grounding, and equipment near the power cable that produces harmonics, such as neon lamps.
• Loose, removed or miswired connections between the indoor and outdoor units.
• The supply voltage at the outdoor unit and the serial signal between the units. For the reverse transfer error the manual then checks the outdoor fan motor and replaces it together with the main PCB, or the main PCB alone; for the forward transfer error it replaces the main PCB.

The voltage and signal checks are made with the unit powered. This is technician work. Fujitsu warns of electric shock and sets a waiting time after power-off before electrical components are touched: 5 minutes in the service manual, 10 minutes in the KPAS installation manual KPAS/KPBS Service Manual SR_AS142ES_09, p. 02-1; KPAS Installation Manual, p. En-2.

Notes for different models

• The other K-series service manuals have their own sections for 11 KNAS Service Manual SR_AS159ES_02, pp. 03-25–03-28; KMAS Service Manual SR_AS134ES_03, pp. 03-22–03-25; KZAS Service Manual SR_AS137ES_03, pp. 03-22–03-25; KTAS Service Manual SR_AS147ES_04, pp. 03-23–03-26; ASLH Service Manual SR_AS143ES_03, pp. 03-20–03-23, and so do the L-series ones LZBS Service Manual SR_AS048EF_02, pp. 03-10–03-13; LZAS Service Manual SR_AS024EF_09, pp. 03-10–03-13; LMAS Service Manual SR_AS058EF_04, pp. 03-3–03-6; LPAS Service Manual SR_AS050EF_03, pp. 03-3–03-6. KNAS drops the “(Outdoor unit)” and “(Indoor unit)” tags KNAS Service Manual SR_AS159ES_02, p. 03-2.
• RLS3-era tables say only “Serial communication error” RLS3 Service Manual, p. 7; their service instructions split it into the two transfer errors RLS3Y Service Instruction, pp. 02-06–02-07; RLB Service Instructions, pp. 02-03–02-04; RLS2 Service Instructions, pp. 02-03–02-04; RLFW1 Service Instruction, pp. 02-03–02-04. RL2 manuals print only the blink counts RL2 Service Manual, p. 12.

Related Fujitsu codes

• 12 “Wired remote controller communication error (Indoor unit)”
• 62 “Outdoor unit main PCB error (Outdoor unit)”
• 97 “Outdoor unit fan motor error (Outdoor unit)”

See also

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• HVAC and furnace tax credits in 2025 and 2026: 25C, 25D, 30C

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