MXZ-SM

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

1. 1 Data pair, not 24 V

   On Mitsubishi MXZ-SM 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 (or box terminals) landing and polarity. No 240 on comm. Never megger the pair.

   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. Isolate: disconnect indoors one at a time (power down first). One bad indoor or one bad run will U4 the outdoor.

   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. If a branch box is present, U4 after all heads are isolated is box-to-outdoor or the outdoor PCB.

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

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

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

   On Mitsubishi MXZ-SM 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, outdoor coil, all indoor coils/filters.

   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. Charge: weigh-in + indoor adders. A starved MXZ looks like U2.

   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. One indoor locked/flooding can load the outdoor — isolate.

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

Outdoor discharge sensor or a real high-discharge event.

1. 1 Which sensor is this code?

   On Mitsubishi MXZ-SM 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 the discharge thermistor.

   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. If the reading is real: overcharge, low airflow across the running indoors, or restriction.

   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 MXZ-SM control input. Do not replace every sensor on the unit.

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

Did not shift or pipe temp 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 MXZ-SM diagram, not a generic heat-pump card. Solenoid voltage during a heat/cool changeover.

   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. Pressure swap at the outdoor. A branch box does not replace the 4-way.

   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.

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

1. 1 Airflow and freeze before you add gas

   On Mitsubishi MXZ-SM 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. Service valves open. Outdoor fan. All indoor blowers that are calling.

   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 + adders. Do not ‘top off’ to a 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. Restriction / drier / one indoor EEV stuck.

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