FieldBench

Mitsubishi

MXZ-SM

Heat pump · MXZ multi-zone outdoor · branch box on some

Mitsubishi MXZ multi-zone outdoor (SM and 3C/4C-class). Several wall or ducted indoors on one outdoor. Some layouts use a branch box — U4 may be box-to-outdoor, not just head-to-outdoor. Charge is weigh-in plus indoor adders. Isolate one bad indoor before you condemn the outdoor.

Not a single-zone MUZ outdoor behind one MSZ-FS. Not City Multi / VRF branch selector. Not Daikin MXS. Not PUZ single-zone P-series.

Walk this in Diagnose

This plate

Ports
Multi-zone · 2–8 indoors depending on SM / 3C / 4C SKU
Branch box
Present on some SM layouts — third board in the comm path
Charge
Factory weigh-in + additional charge per indoor / line-set adder
Example SKU
MXZ-SM36NL
Indoors
MSZ-FS, MSZ-GL, SEZ, SVZ-class — each indoor has its own chart

This board

MXZ outdoor PCB (+ branch box when installed)

  • Outdoor LED / service mode plus each indoor’s code. They will not always match — write outdoor, box, and indoor.
  • U4 on a branched system may be box-to-outdoor. Killing power to one indoor is the isolation test.
  • E8/U2 is outdoor overcurrent / high load. F3 is discharge. EA is 4-way. High/low pressure protect is the outdoor pressure path — read the MXZ card for the exact token on this date.

Sequence

  1. Call. Any indoor requesting capacity wakes the outdoor inverter. Other indoors may be off — that is normal.
  2. Branch box. On boxed systems the outdoor talks to the box, the box talks to the heads. U4 can be either hop.
  3. Defrost. Outdoor coil strategy serves every indoor. One indoor in heat while another is in cool is a mode-conflict, not a furnace W.

Faults on this IFC

  • U4

    Communication

    lockout

    Transmission failed. On MXZ this may be outdoor–indoor or outdoor–branch box.

    • S1/S2/S3 (or box terminals) landing and polarity. No 240 on comm. Never megger the pair.
    • Isolate: disconnect indoors one at a time (power down first). One bad indoor or one bad run will U4 the outdoor.
    • If a branch box is present, U4 after all heads are isolated is box-to-outdoor or the outdoor PCB.
    • Do not assume Daikin pinout.
  • E8 / U2

    Overcurrent / high load

    lockout

    Compressor or outdoor protection — high amps, high discharge load, or drive current.

    • Winding/ground, outdoor coil, all indoor coils/filters.
    • Charge: weigh-in + indoor adders. A starved MXZ looks like U2.
    • One indoor locked/flooding can load the outdoor — isolate.
  • F3

    Discharge thermistor

    warn

    Outdoor discharge sensor or a real high-discharge event.

    • Ohms the discharge thermistor.
    • If the reading is real: overcharge, low airflow across the running indoors, or restriction.
  • EA

    4-way valve

    lockout

    Did not shift or pipe temp disagrees.

    • Solenoid voltage during a heat/cool changeover.
    • Pressure swap at the outdoor. A branch box does not replace the 4-way.
  • Pressure protect

    High / low pressure protect

    lockout

    Outdoor high- or low-pressure protection (token is on the MXZ door card for this revision — do not invent a City Multi number).

    • Service valves open. Outdoor fan. All indoor blowers that are calling.
    • Weigh-in + adders. Do not ‘top off’ to a piston SH chart.
    • Restriction / drier / one indoor EEV stuck.

Workflows

Isolate one bad indoor

MXZ U4 or the outdoor will not stay running.

  1. Map every indoor SKU (FS vs GL vs ducted). Each head has its own chart for P1/P6.
  2. Power down. Leave one indoor connected. Restore power. Repeat. The indoor that brings U4 back is the path — head, cable, or that port on the box.
  3. Branch box present? After all heads are off, a remaining U4 is box-to-outdoor.
  4. Do not replace the outdoor inverter first. MXZ outdoor boards die from 240 landed on comm, not from one dirty filter.

Charge an MXZ

U2 / pressure protect / poor capacity.

  1. Recover and weigh. Factory charge + line-set + each indoor adder per the MXZ SM / 3C chart.
  2. There is no piston superheat chart that saves this.
  3. Additional indoor added last summer without extra charge is the classic undercharge.

Test points

Comm hops
Outdoor–box and box–indoor are separate. U4 needs to be localized.
Charge
Weigh-in + indoor adders. Write every indoor model before you calculate.

Gotchas

  • MXZ-3C / MXZ-4C share this multi-zone idea. Port count and branch-box presence still come from THAT plate.
  • MSZ-FS and MSZ-GL on the same MXZ is normal. Do not use the FS i-see playbook on the GL head.

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