Communication Error - Help me troubleshoot Fujitsu Halcyon 36 BTU Mini Split

1. This happened the day after I "mouse proofed" where the lineset enters the house buy cleaning out the passthrough, packing with steel wool and spray foaming in.
2. This may or may not have been the cause...it could have been an unlikely coincidence.
3. Once the error started to happen, I opened up the outside unit, and also found a rodent nest (which I removed)
4. I can see some corrosion on one of the boards where the rodent nest was located.

1. Turned off breakers and let sit for 30 minutes or more (up to 3 months)
2. Checked all the fuses for continuity (all good)
3. Tested the voltage coming in (240)
4. Tested the continuity of the wires going to 1 indoor unit (all good).
   1. Disconnected the other 2 indoor units, let sit for 30 minutes disconnected and retest with only the 1 confirmed wire set/indoor unit tested
   2. Moved the control wires from Unit B to Unit A connections on the outdoor unit for the good tested indoor unit

1. Hope the smart folks on the Garage Journal can help me test
2. Call and HVAC place (my original installer is no longer in business)
3. Buy new control boards and replace myself (one of the boards is close to $1000)
4. Just replace the outdoor unit

• 1501HUE-C1
  • SCE 200109 R A50
• 1502HUE-FLO
  • SK 20.01.09r
• 1502HUE-TRI (or TR1) or just TR - this is the board with corrosion/mouse damage (seen as slight discoloration)
  • SCE 200114 R A16

That is what I tested. I verified one indoor unit has good communication wiring and removed the other units. No changes, and problem still exists.

This happened the day after I "mouse proofed" where the lineset enters the house buy cleaning out the passthrough, packing with steel wool and spray foaming in.

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Step 2: Live Voltage Testing (Isolating the Board)​

Fujitsu systems route 240VAC across Terminals 1 and 2 to power the indoor units, while Terminal 3 acts as the data line, generating a fluctuating DC pulse relative to Terminal 2 (and Ground).

To pinpoint whether the indoor or outdoor board is at fault, perform the standard serial signal test:

1. Check Base Power: With the breaker back on and the system calling for operation, measure AC voltage across Terminal 1 and Terminal 2. You should read a stable 208/230VAC.
2. Measure Communication Pulse: Switch your multimeter to DC Voltage. Measure between Terminal 3 and Terminal 2 (or Terminal 3 to Ground) at the outdoor unit.
   • Normal Behavior: You should see a dynamic, fluctuating DC voltage bouncing rapidly anywhere between 10VDC and 70VDC+ (some meters register this as a wide swing up to 100V). This indicates the microprocessors are actively modulating the data lines.
   • Abnormal Behavior (Static Voltage): If the voltage is completely dead (0VDC) or stuck solid at a flat line (like a fixed 30VDC or 50VDC with absolutely no fluctuation), the communication loop is broken.

Step 3: Isolating the Outdoor PCB​

To prove the outdoor board is the culprit rather than an indoor unit pulling down the signal:

1. Kill the power again.
2. Disconnect the Terminal 3 wires for all three indoor units from the outdoor terminal block. (Keep them organized so you know which goes to which zone).
3. Power the system back up.
4. Measure DC voltage directly on the outdoor unit's bare Terminal 3 to Terminal 2.
   • If you still get 0VDC or a completely frozen, unfluctuating DC voltage, the communication circuit on the Outdoor Main PCB is dead. The board will need to be replaced.
   • If you suddenly see a healthy, rapidly fluctuating DC voltage jump back to life on the bare terminals, the outdoor board is working. One of your indoor units or its corresponding traveler wire has a short/ground fault that is dragging down the entire communication bus. You can hook the zone 3-wires back up one by one to see which specific line kills the voltage.

Summary of Likelihood​

Given that this is a 2020 build and all three indoor units dropped communication simultaneously, the most common failure point is the outdoor main controller PCB blowing its communication optocouplers or transient voltage suppressors (often triggered by line utility spikes or local lightning storms). If your isolated terminal test in Step 3 confirms a dead, flat DC voltage, replacing the outdoor board is your definitive fix.

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462867aa said:

My unit was mismarked from the factory. Same error. Look at connections between indoor and outdoor units.

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dcg9381 said:

I'm no HVAC guy, but you messed with something and it changed. The "coms" on this are highly sensitive to wire changes and impedance changes. When you "mouse proofed" the lineset, did that also touch the coms wire?

My ductless have dedicated coms lines, if that's the case here, I'd run a cat 6 cable all the way around (but I don't have multi-head).

I got some tech help on it, this is what is suggested for that model:

Click to expand...

dcg9381 said:

I'm no HVAC guy, but you messed with something and it changed. The "coms" on this are highly sensitive to wire changes and impedance changes. When you "mouse proofed" the lineset, did that also touch the coms wire?

My ductless have dedicated coms lines, if that's the case here, I'd run a cat 6 cable all the way around (but I don't have multi-head).

I got some tech help on it, this is what is suggested for that model

Click to expand...

1. With one head unit connected (tested good on continuity of control wires).
   1. Flat 50v DC
2. Disconnected:
   1. 7v dc