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Models
Mitsubishiminisplit

MSZ-FS

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.

Diagnose this modelInstall notesBoard cross-refField wiring

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.

Indoor service mode + outdoor LED. Power cycle clears live, not always history.

Universal swap

No universal

Mini-split indoor PCB. No WR/ICM furnace board applies.

White-Rodgers

—

ICM

—

Honeywell

—
  • Indoor furnace universals (50A55, 50M56U, ICM282B, ICM2811). This is not an IFC.
  • Do not ‘adapt’ a furnace board to an outdoor inverter or defrost PCB.

After you pull the dead card

  1. 1OEM Mitsubishi indoor control. This is not a gas IFC.

Field wiring

Written landings for Mitsubishi MSZ-FS. Not a factory schematic.

Mitsubishi M / MXZ · S1 S2 S3
S1 / S2 / S3R-410A

M-series wall and MXZ multi-zone. S1/S2 power, S2/S3 comm on most M-series. Not Daikin P1/P2 and not 24V thermostat cable.

Written circuit description only — FieldBench does not redraw maker schematics. Example: R to W closes on a heat call; verify 24V at W1 before you change the IFC.

Open manufacturer PDF

This indoor

Hyper-Heating wall. S1/S2/S3 to the outdoor or MXZ.

S1 / S2 / S3

Head to outdoor, or outdoor to branch box on MXZ.

Indoor head

Voltage on S1/S2 (power) vs S2/S3 (comm) per THAT outdoor diagram.

  • S1PowerLine
  • S2Power / comm commonLine
  • S3CommData

Outdoor / branch box

Same three. Never megger the pair. Isolate heads one at a time on U4.

  • S1PowerLine
  • S2CommonLine
  • S3CommData

Landing

  • LineHead S1outdoor S1

    Line / power — not 24V

  • LineHead S2outdoor S2

    Common

  • DataHead S3outdoor S3

    Comm. No 240 on this pin.

Prove it

  • S1/S2 power vs S2/S3 comm per the diagram on THAT outdoor.
  • U4: isolate indoors (power down first).

Do not

  • Do not treat this as 24V R/C/Y.
  • Do not assume Daikin P1/P2 or Carrier ABCD.
  • Never megger S2/S3.

S1/S2/S3 is the published M-series / MXZ landing.

Sequence

  1. 1

    Call

    Indoor board requests capacity on the comm pair. Outdoor inverter ramps. No contactor on current M-series.

  2. 2

    Defrost / H2i

    FS/Hyper-Heating has its own outdoor coil strategy. EA is the 4-way if it does not shift.

  3. 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

Step 1, then 2, then 3. Expected reading and the fail branch sit under each check.

U4

Communication

lockout

Indoor–outdoor transmission.

  1. 1 Data pair, not 24 V

    On Mitsubishi MSZ-FS indoor/outdoor comm is a data pair (AB, S1/S2, U, ComfortBridge, ComfortLink — whatever THIS plate uses). It is not a 24 V Y. Check polarity, continuity, and that nobody landed it on R/C. S1/S2/S3 landing, polarity, no 240 on comm, outdoor PCB LED.

    Expect: Pair continuous, not shorted to chassis, polarity per the diagram.

  2. 2 Power both ends, then the outdoor status

    Indoor transformer and outdoor control both alive. Read the outdoor LED / 7-seg. Water in the outdoor control is the usual dead-end after a storm. On an MXZ pair, U4 may actually live on the outdoor / branch-box chart — isolate this head.

    Expect: Both controls powered. Outdoor showing a comm or address state, not a blank board.

  3. 3 Address / bias, then the control

    Communicating plates need a matching indoor address and sometimes a terminating bias. Swap is last: known-good pair + power both ends + correct address.

    Expect: Indoor sees the outdoor. No comm code after a power cycle.

P1 / P2

Indoor thermistors

warn

Return or pipe sensor.

  1. 1 Which sensor is this code?

    On Mitsubishi MSZ-FS P1 / P2 is a specific thermistor — indoor return/pipe, outdoor coil, discharge, or ambient. Unplug that sensor and ohm it vs the OEM temperature/resistance chart. Ohms vs M-series chart, connector on indoor board.

    Expect: Ohms move with temperature. Not open, not shorted.

  2. 2 Seating and the harness

    A sensor that ohms good on the bench and fails in the unit is seating or a rubbed harness. Reseat in the well / on the coil, check the plug at the board.

    Expect: Sensor fully in the well. Harness not pinched.

  3. 3 Board input last

    Known-good sensor + known-good harness and the code stays = Mitsubishi MSZ-FS control input. Do not replace every sensor on the unit.

    Expect: Code clears with a known-good sensor at the board.

P4 / P9

Drain / float

lockout

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

  1. 1 Float and the pan

    On Mitsubishi MSZ-FS a drain / float code is water until proven otherwise. Pan full? Float stuck? Secondary drain piped? Pump, float, pitch. Not an i-see code.

    Expect: Pan dry after you clear it. Float moves freely.

  2. 2 Trap and the pump

    Indoor trap primed and flowing. Condensate pump actually pumping, check valve not stuck. On a furnace, a drowned collector will also throw PS codes — clear both.

    Expect: Trap flowing. Pump discharges. No water in the collector.

  3. 3 Then the switch circuit

    If the pan is dry and the code stays, ohm the float / condensate switch and the IFC input. Do not jump a wet-switch input as a repair.

    Expect: Switch closed when dry. Code gone.

P6

Freeze / overheat

warn

Coil out of band.

  1. 1 Airflow and freeze before you add gas

    On Mitsubishi MSZ-FS low pressure / freeze is indoor airflow or a starved coil until proven otherwise. Filter, blower, dirty A-coil, closed supplies. In heat mode, a iced outdoor coil is the same family. Filter, fan, charge, sensor.

    Expect: Indoor air moving. Coil not iced. OD coil clear in heat.

  2. 2 SH / SC after a thaw

    Thaw fully. Then SH and SC vs the metering device. Low charge ices a piston. A TXV stuck closed ices with high SH. A restriction downstream of the ports fakes a low-charge chart. Weigh-in — no piston SH chart.

    Expect: SH/SC in range for this refrigerant and device after a stable 10-minute run.

  3. 3 Leak, TXV, then the LPS

    Weigh-in if the chart says undercharge. Soap / electronic leak on joints and the coil. LPS that opens on a known-good charge and a clear coil is the switch — momentary test only, then restore.

    Expect: Charge on the nameplate. LPS closed at a normal suction for this OD ambient.

F3

Discharge thermistor

warn

Outdoor discharge.

  1. 1 Which sensor is this code?

    On Mitsubishi MSZ-FS F3 is a specific thermistor — indoor return/pipe, outdoor coil, discharge, or ambient. Unplug that sensor and ohm it vs the OEM temperature/resistance chart. Ohms. If the reading is real, high SH / low charge / overcharge per mode.

    Expect: Ohms move with temperature. Not open, not shorted.

  2. 2 Seating and the harness

    A sensor that ohms good on the bench and fails in the unit is seating or a rubbed harness. Reseat in the well / on the coil, check the plug at the board.

    Expect: Sensor fully in the well. Harness not pinched.

  3. 3 Board input last

    Known-good sensor + known-good harness and the code stays = Mitsubishi MSZ-FS control input. Do not replace every sensor on the unit.

    Expect: Code clears with a known-good sensor at the board.

E8 / U2

Overcurrent / high load

lockout

Compressor or outdoor protection.

  1. 1 Let it cool, then read the drive LED

    On Mitsubishi MSZ-FS overcurrent / overheat is often a dirty coil, a failed OD fan, or a locked compressor — not a dead inverter. Clear airflow, confirm the OD fan, and read the drive LED before you order a control. Winding/ground, coil, charge. Weigh-in.

    Expect: OD fan running. Coil clean. Drive LED decoded from THIS outdoor’s card.

  2. 2 Voltage and compressor windings

    Incoming voltage in range (this plate’s line-voltage code). Compressor windings even, no ground. A shorted compressor will kill a new drive.

    Expect: Balanced windings. No winding to ground. Line voltage stable under load.

  3. 3 Charge and the drive last

    A starved or flooded compressor will overcurrent. Confirm SH/SC and charge weight. Only then condemn the inverter.

    Expect: Charge on the nameplate. Drive code gone after a cool-down and a clean coil.

EA

4-way valve

lockout

Did not shift or sensor disagrees.

  1. 1 Confirm O/B is energized the way THIS outdoor expects

    Most US splits energize O in cool. Some Rheem/Ruud energize B in heat. Read the Mitsubishi MSZ-FS diagram, not a generic heat-pump card. Solenoid voltage during the call, pressure swap, suction temp.

    Expect: Correct 24 V at the solenoid for the mode that is failing.

  2. 2 Listen for the valve, then measure

    A healthy valve slams. 24 V at the solenoid on the shift call? Voltage + no slam → valve mechanically stuck. No voltage → stat / defrost board / IFC.

    Expect: Solenoid voltage present. Pressures swap after the slam.

  3. 3 Stuck valve vs a defrost board that never leaves cool

    A defrost board stuck in defrost looks like ‘no heat’ with the OD fan off. If the valve is energized and pressures never swap, recover and replace the valve — do not beat on it as a repair.

    Expect: Mode change on the gauges. OD fan on in heat except during a real defrost.

Workflows

U4 on MSZ-FS

Indoor powered, outdoor silent.

  1. 1 Prove the call and the live code

    Voltage on S1/S2 (power) vs S2/S3 (comm) per the diagram on THAT outdoor. Do not assume Daikin pinout.

    Expect: Write whatever Indoor + outdoor LED codes is showing. Indoor service mode + outdoor LED. Power cycle clears live, not always history.

    If fail: No call at the IFC → stat / harness. Do not change gas parts.

  2. 2 Isolate the first failed step in the sequence

    Single-zone: indoor board, outdoor board, the three-wire. Never megger the comm pair.

    Expect: Defrost / H2i: FS/Hyper-Heating has its own outdoor coil strategy. EA is the 4-way if it does not shift.

  3. 3 Confirm with a number, then the next part

    If the outdoor is MXZ, stop and use the MXZ-SM chart — one bad indoor or the branch box can U4 the outdoor.

    Expect: 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.

    If fail: Stay on this plate. The out-of-range reading is the part.

    If pass: Reading in range → the next step in this workflow is the repair.

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

Install this plate

Install MSZ-FS as an M-series Hyper-Heating wall. S1/S2/S3 only. i-see sensor aims into the room.

Wall indoor · S1/S2/S3 · no R/W/Y · pair MUZ single-zone or MXZ multi-zone · MSZ-FS06NA / 09NA typical

Mount & piping

  • Wall sleeve, flare torque, nitrogen braze on any extended lines. Condensate gravity or mini-pump — not a furnace trap.
  • i-see sensor must see the room. A soffit or cabinet that blinds it is an install miss unique to FS / FH.
  • Line-set length and additional charge: use the outdoor I/O (MUZ vs MXZ). Indoor adders live on the outdoor chart.

Wiring

  • S1/S2/S3 (or A/B on some). No conventional 24V R/W/Y. Stranded, correct polarity — swapped S2/S3 is a classic no-comm.

First fire

  1. 1Address / pair per the outdoor. Cool then Hyper-Heat. Confirm i-see sweep.
  2. 2Indoor P-codes live here. Outdoor U-codes live on MUZ / MXZ.

Do not on Mitsubishi MSZ-FS

  • Do not apply MSZ-GL / GS install (no i-see, no FS Hyper-Heating extras).
  • Do not land 24V thermostat wire on S1/S2/S3.

Maker literature

FieldBench does not host manufacturer files. Each link opens the maker’s own literature page in a new tab.

  • MSZ-FS product pageProduct pageDeluxe Hyper-Heating wall indoor · 3D i-see · 6–18 kBtu.Open manufacturer PDF
  • MitsubishiPro technical libraryLiterature searchSubmittals, I/O, and service manuals for MSZ / MXZ. Public site is product-only.Open manufacturer PDF
  • MitsubishiPro technical libraryLiterature searchSearch Mitsubishi MSZ-FS on the maker’s literature page.Open manufacturer PDF