Rapid intervention for an indoor unit stuck in ventilation mode
At AirGreen , we regularly receive calls for partial operation problems on Fujitsu wall-mounted heat pumps , including units that appear “alive” (lights on, shutters open) but which produce neither cold nor hot air .
This is exactly the scenario encountered during this service call in Ville Mont-Royal , in a recently renovated duplex.
The customer reported that their Fujitsu Inverter system had been working intermittently for several days. The indoor unit would turn on, but no air would circulate, even in fan mode. This is a typical situation caused by a blocked circuit board or a communication failure between the units .
Step 1: Diagnosis of the indoor unit
Upon our arrival, our technician carried out a complete visual inspection .
Upon removing the front panel, we found that the directional shutter was stuck in the half-open position , and a flashing LED on the circuit board signaled a communication error.
Using our Fujitsu code reader, we quickly identified error code 09 — “communication error between indoor unit and outdoor unit” .
Possible causes include:
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A cut or loose communication cable (DC signal) .
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Unstable power supply on the L/N terminal.
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A faulty internal interface card .
To confirm, we opened the cover of the electrical box located on the right side of the unit (visible in the photo), where the main connection terminals are located.
Step 2: Electrical Check and Voltage Measurements
Using a calibrated digital multimeter, we measured:
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230V between L and N: correct voltage.
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0V on the communication terminal (S) : signal absent.
This confirmed a DC communication failure , probably caused by a faulty internal control board or a loose connection in the cable connecting the two units .
After a thorough inspection of the wall wiring, we found a slightly loose communication terminal , a small defect but enough to interrupt the dialogue between the electronic boards.
Step 3: Repair and Recommissioning
We have proceeded to:
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Tighten and clean the connections on the communication terminals.
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Test the cable continuity between the two units (measured resistance: 0.2 Ω — compliant).
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Reset the Fujitsu system by powering off for 3 minutes.
Upon restart, the error light disappeared and the indoor unit restarted normally .
The fan started running again, and after 2 minutes the unit was producing warm air—evidence that the outdoor compressor was receiving the control signal again.
Step 4: Preventive Cleaning and Final Testing
As the unit had not been opened for several seasons, we took advantage of the intervention to carry out a quick cleaning of the filters and the room temperature sensor .
These dirty sensors can sometimes cause incorrect temperature readings and prevent the heating or cooling cycle from starting.
We then checked:
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Blown air temperature: 42°C in heating mode .
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Stable voltage on the communication bus: 34V DC .
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Consumption as expected (2.1 A).
Everything worked perfectly.
Step 5: Maintenance tips to extend the life of the system
We advised the client:
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Have the electrical connection between units inspected annually, especially after renovations or wall painting.
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Have the filters cleaned monthly during periods of intensive use.
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Schedule a professional AirGreen service once a year to completely clean the evaporator, fan and drain.
These simple actions help prevent costly electronic breakdowns and ensure optimal performance of the Fujitsu system.
Result: comfort restored and reliability regained
In less than an hour, our team has:
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Diagnosed the exact cause of the fault (loose connection).
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Repaired the communication circuit.
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Tested system performance and validated electrical parameters.
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Returned the Fujitsu heat pump to full service.
The customer found stable and quiet heating , perfectly suited to Montreal winters.
Once again, our rigor and methodical approach made it possible to resolve a complex electronic failure without unnecessary replacement .
AirGreen: your HVAC reference in Montreal and on the North Shore
At AirGreen , each intervention combines precise diagnosis, transparency and efficiency .
Our technicians are trained on all brands of wall-mounted heat pumps — Fujitsu, Midea, Gree, Sharp, Lennox, Zephyr, Goodman, Trane — and operate throughout Montreal, Laval, Longueuil, North Shore and South Shore .
Trust our expertise for:
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Communication failures between units.
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Faulty electronic cards .
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Complete cleanings and preventive maintenance .
AirGreen guarantees professional, fast, and long-lasting service for all your residential HVAC needs.