FieldBench

Mitsubishi

MSZ-FS

Mini-split · M-series Hyper-Heating wall indoor · i-see sensor

Mitsubishi Hyper-Heating wall indoor (MSZ-FS, previous-gen MSZ-FH). Faults are indoor LED / outdoor LED (U4, P1, P6, E8, F3, EA). There is no 24V W sequence and no furnace 7-seg. i-see is FS/FH only — do not look for it on GL.

Not MSZ-GL / MSZ-GS (cheaper M-series wall, no i-see). Not MXZ-SM / MXZ-3C outdoor (those are the multi-zone outdoor chart). Not PUZ/SVZ P-series ducted. Not a 24V split with a furnace IFC. Not Daikin VRV.

Walk this in Diagnose

This plate

FS / FH
Hyper-Heating wall indoor · i-see sensor
Example SKU
MSZ-FS06NA / MSZ-FS09NA
Wiring
S1/S2/S3 (or A/B on some) · no conventional R/W/Y
Pair
Single-zone outdoor or an MXZ multi-zone outdoor (use the MXZ chart for the box)

This board

Indoor + outdoor LED codes

  • Indoor flashes a letter+number (P1, P6, U4). Outdoor may flash a different code for the same fault — write both.
  • U4 is comms on the S1/S2/S3 (or A/B) pair. High voltage on that pair cooks both boards.
  • P6 is coil freeze/overheat protection — airflow or charge, not a furnace limit.
  • i-see sensor faults are FS/FH only. If the head is a GL, leave this page.

Sequence

  1. Call. Indoor board requests capacity on the comm pair. Outdoor inverter ramps. No contactor on current M-series.
  2. Defrost / H2i. FS/Hyper-Heating has its own outdoor coil strategy. EA is the 4-way if it does not shift.
  3. i-see. FS/FH occupancy / floor sensor. Absent on GL — do not diagnose a missing i-see on a GL head.

Faults on this IFC

  • U4

    Communication

    lockout

    Indoor–outdoor transmission.

    • S1/S2/S3 landing, polarity, no 240 on comm, outdoor PCB LED.
    • On an MXZ pair, U4 may actually live on the outdoor / branch-box chart — isolate this head.
  • P1 / P2

    Indoor thermistors

    warn

    Return or pipe sensor.

    • Ohms vs M-series chart, connector on indoor board.
  • P4 / P9

    Drain / float

    lockout

    Cassette/ducted more than wall, but FS can still show a drain accessory fault.

    • Pump, float, pitch. Not an i-see code.
  • P6

    Freeze / overheat

    warn

    Coil out of band.

    • Filter, fan, charge, sensor.
    • Weigh-in — no piston SH chart.
  • F3

    Discharge thermistor

    warn

    Outdoor discharge.

    • Ohms. If the reading is real, high SH / low charge / overcharge per mode.
  • E8 / U2

    Overcurrent / high load

    lockout

    Compressor or outdoor protection.

    • Winding/ground, coil, charge. Weigh-in.
  • EA

    4-way valve

    lockout

    Did not shift or sensor disagrees.

    • Solenoid voltage during the call, pressure swap, suction temp.

Workflows

U4 on MSZ-FS

Indoor powered, outdoor silent.

  1. Voltage on S1/S2 (power) vs S2/S3 (comm) per the diagram on THAT outdoor. Do not assume Daikin pinout.
  2. Single-zone: indoor board, outdoor board, the three-wire. Never megger the comm pair.
  3. If the outdoor is MXZ, stop and use the MXZ-SM chart — one bad indoor or the branch box can U4 the outdoor.

Test points

S1/S2/S3
Power vs comm per the MSZ/outdoor diagram. Not 24V thermostat-cable logic.
Charge
Weigh-in on the paired outdoor. Additional indoor adders only if that outdoor is MXZ — see the MXZ chart.

Gotchas

  • MSZ-GL is the cheaper wall — similar letter codes, no i-see. Do not diagnose FS-only features on a GL.
  • MXZ-SM / MXZ-3C / PUZ / SVZ are different families. They are not aliases of this head.
  • Do not apply Goodman 3-flash or Carrier 31 to these LEDs.

Related plates

  • mitsubishi-mxz-sm — Multi-zone outdoor this head is often paired with. Branch box and indoor adders live there.
  • mitsubishi-msz-gl — Cheaper M-series wall. Same letter-code idea, no i-see, no Hyper-Heating extras.