It is a good practice to make sure all of your dip switches are set to the factory position on all of your flow selector boxes during installation. If the dip switches are not set in the factory position you will have L11 and E17 error codes to follow. You can verify the factory dip switch settings by looking at access door on the flow selector box. The L11 and E17 error codes can also be caused by wiring issues, good wiring practices can prevent having problems.


Changing dip switches on your flow selectors is not necessary because all of your flow selector addresses and port numbers are set using and codes 00FE and 0105 on your thermostat. Making sure your flow selector address and your flow selector port address is correct will make the start up process much easier. If care is taken and you have a good understanding of the wiring and installation of a VRF system the start up process can be close to seamless.


L11- Detected indoor unit address, Flow selector unit not connected.


E17- Communication trouble between indoor unit and flow selector unit.

Meter leads on the DFT position 1 and the C terminal.


Schematic: The circles represent where meter leads were making contact for testing.

The Pipe Sensor:

Copper pipes and a blue wire connected to a small electrical component in a mechanical setup

The pipe sensor monitors the temperature of the water going to a 2-pipe FCU, also known as changeover sensor, changeover switch, Aquastat, among others:


  • If the water is hot (> 83 F), it signals the thermostat that the system is in heat mode;
  • If the water is cold (< 65 F), it signals the thermostat that the system is in cooling mode;
  • User configurable for Open or Closed activation
  • Compatible with both on-Cool / on-Heat sensors (determined on 1st run setup)
  • Compatible with dry-contact pipe sensors (on/off), not analog. (ex. 10K ohm thermistor)
  • Dehumidify with Fan (Yes or No): when Dehum is active this setting specifies if you want the furnace/air-handler fan to run as well. This will activate (G) output on the t-stat at the same time. This can work independent of a cooling cycle.
  • Min Runtime Delta (2-10%): to reduce short cycling, the dehumidifier will run beyond the desired humidity by the selected amount (%). Default is 5%.
  • Dehumidify in Heat Mode (Yes or No): If humidity is a concern during the heating season and have a standalone Dehumidifier, this setting can be used to activate the Dehumidifier and will work independent of a heating cycle.
  • Dehumidifier Active (Open or Closed): this setting is dependent whether the HVAC system or standalone Dehumidifier requires it to be open or closed while active. This setting is part of the 1st run setup but can change afterwards in the Dehumidifier menu.

Find the service literature for your specific outdoor unit. The compressor model numbers, and ohm values are published in every service manual for these units. Match your numbers to the chart provided on the back of the PBA and table below.


Or, if you prefer the hard way, just check the numbers on the compressor. Unfortunately, the model tags are facing the inside of the unit. You can try to get your camera phone in there and get a photo or use your mirror from your torch set and a flashlight to see back there and verify the model. Then set the SW1 dipswitches accordingly. See table below.

Crowded multicolor wire plug connected to green circuit board terminals labeled A, B, C, D

Once signed into your account, touch “Connect to Outdoor Unit or Furnace”. This will take you to the “Connect to Equipment” screen. Select “Outdoor Unit (Bluetooth)” and the app will search for equipment.

2. Condenser: Half the magic happens here as high-pressure vapor pumped from the compressor rejects heat until it has changed state and condensed into a high-pressure sub–cooled liquid.

Jesse Van Atta

VRF Quality Assurance Manager Gulf Coast District

July 10, 2023

The Dreaded P4 Code

Have you ever shown up to a service call on a Carrier/Bryant/WeatherMaker (Midea) ductless system and found it locked out on a P4 or PC04 error code showing on the indoor display? Did the code return after a power cycle? Did the unit attempt to run? This article will help you understand what these codes mean and how to troubleshoot them effectively.


When working on any system, it is important to know the model number. These codes can mean different problems depending on the equipment, and where the error is displayed. In this article, I am referring to the following outdoor models:


  • (P4) 38MAQ, 38MBR, 38MHR, 38MPRA
  • (PC04) 38MAR, 38MPRB


Tips and Tricks: The following diagnostics can also apply to P6 and PC00 codes for IPM Protection


What do these codes mean? P4 and PC04 are Inverter Compressor Drive errors. Even though they sound specific, these codes can be generated from a broad spectrum of component failures and operating issues within the system.


Know your Voltage

Read L1 to L2. What is the actual line voltage vs the minimum and maximum listed on the data plate? Read each leg to ground. Are they balanced? Check the main breaker, next the disconnect, and then the equipment. Was there any drop or difference in the voltage?

Wiring diagram for an AC unit with L1, L2, G lines and voltage/phase imbalance limits.

Replacing a part may remedy a symptom, but not necessarily the cause. Dirty power issues can be overlooked when components have failed. If left unchecked, voltage that is too high, too low, or imbalanced can make for recurring issues.

Understand the Inverter Components and Their Purpose

Diagram of ductless technology with blue and red airflow paths through equipment and filter stages

The first stop in the inverter power circuit is the bridge rectifier. Its purpose is to convert the AC line voltage to DC. In essence, it flips the bottom half of the sine wave to the top. The formula for DC output is 1.414 x AC line voltage. Next in the circuit are the reactor and capacitors. These components pull tension on and flatten out the sine wave. Then the IPM (Intelligent Power Module) or IGBT (Insulated Gate Bipolar Transistor) uses this voltage to send precisely controlled frequency to the compressor and DC fan motor(s).


After you have verified a good power supply to the equipment, perform a visual inspection of the wiring and circuitry. Some issues are more obvious than others.

Close-up of electronic circuit board with yellow capacitors and copper wiring inside a metal housing

Lightning prevails over circuitry

Close-up of electronic circuit board with wires, coils, and yellow components inside a device

Tree frog had a bad day

Check the Reactors and Capacitors

These components are integral to the inverter circuit. Capacitors can usually be found bad on a visual inspection. Failures in reactors are uncommon, and they can be easily overlooked. Reactors should ohm very close to zero resistance. They should never be open or have any continuity to ground.

Close-up of an electrical component with wires and a label inside a metal appliance.
Table showing resistance 0–1 Ω and voltage to ground: DC 310V standby, DC 280V compressor running

Ohm the Windings from the Wire Harness Connector

Check the Compressor

Close-up of a black shoe with a brown speckled insole and green trim, next to a black object.

Has the sound blanket turned from green to brown? This is a sure sign it has overheated or possibly become demagnetized. Ohm the compressor windings. Remember, this is a three-phase motor. Your measurements should be balanced and less than 2.25 ohms. Nothing should read open or continuity to ground.

Blurred spreadsheet row with black and blue cells, divided into several labeled columns.

Ohm Values for the Windings of Select Model Compressors

Wiring diagram beside hands using a multimeter to test connected wires

Ohm the Windings from the Wire Harness Connector

Check the DC Fan Motor(s)



Does it spin freely? If not, then the bearings have failed. If yes, then ohm the windings from the UVW connector. It will be a much higher resistance than the compressor but should still be balanced between any two pins. It should not be open or grounded. On units with two motors, perform the process with each.

Three side-by-side shots of a digital multimeter testing wiring, labeled Blue to Yellow, Blue to Red, and Yellow to Red.

If the fan motor windings read correctly, further investigation may be necessary. Turn on power to the unit. Some motors use feedback for speed recognition. During standby, 5 VDC should always be present on the 4 & 5 pins of the feedback connector.

Color-coded pinout chart showing signal labels for orange, gray, white, pink, black, red, blue, and yellow wires

If it is not present, you have a shorted PC board. If it is present, then progress to the next step. Rotate the fan by hand and measure pins 1 & 5, 2 & 5, and 3 & 5. If any negative voltage is displayed, then replace the motor.


Check The Inverter (IPM)

IPM Community Church weekly schedule table with service times and locations in black-and-white.

Learning to find all these check points on the back of a control board takes time and experience. Every board is different. If they do not have to be returned for warranty purposes, save your failed parts for practice and study tools.


Communication Is Key

Bad communication between the main PC and the Inverter can cause a P4 code. Check the wired connectors for poor alignment. This can happen when components are replaced, and someone pulled on the wires instead of the connector. Loose connections can wreak havoc on a communication circuit.

Close-up of a 6-pin wire connector with colored wires labeled red, brown, blue, orange, yellow, and white.

Example of a misaligned connector

Review of Possible Causes for P4 and PC04 Codes


  • High, Low, or Imbalance Voltage
  • Shorted PC and/or IPM Boards
  • Problems with the Compressor
  • Problems with the DC Condenser Fan Motor(s)
  • Faulty Communication between Main PC and IPM
  • Miswiring and Loose Connectors
  • Liquid in the Refrigerant (remote possibility)


Hopefully, the knowledge from this article will take some of the dread out of diagnosing a unit with these error codes and help you be more efficient when working with ductless products.

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion.

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