Find the Break in a Communication Error
Check the signal between the indoor and outdoor units step by step, so you know whether it's a board or the wire.
System Diagnostics
Know It's the Board Before You Order One
Ductless faults are easy to misread. ACLogics walks you through communication errors, inverter boards and expansion valve problems so you don't replace parts that weren't broken.
FAULT SYMPTOM
Communication error (E6/U4)
CHECK 01
Measure DC voltage between terminal S1 and S3. Is it pulsing?
Pulsing between 0V and 24VDC
DIAGNOSIS
Electrical noise from unshielded wires. Verify proper S3 shield grounding at outdoor unit.
Stuck solid at 0V or 24VDC
CHECK 02
No pulsing detected. Measure voltage level.
0VDC — No power from outdoor board24VDC Solid — Indoor sending, no reply
Complete diagnostic tree — every branch, every fault path
Try ACLogics Free — iPhone & AndroidWhat You'll Stop Guessing About
Know what failed before you order the part.
Spot a Bad Sensor Fast
Compare thermistor resistance against temperature, so a drifted sensor doesn't get blamed on the board.
Test Before You Condemn the Inverter Board
Follow safe checks on the high-voltage side before you order an expensive board.
How a Call Goes
01
Enter the Code
Enter the error code or blink pattern for context on the unit you're working on.
02
Narrow It Down
Follow guided checks to tell the indoor board, outdoor board and compressor side apart.
03
Confirm It's Running
Check communication is back and the compressor ramps up properly.
Frequently Asked Questions
I have a communication error (E1/E7). Is it always the outdoor board?
No. It can be the indoor board, the wire harness, or even a reactor. We guide you through checking the S-terminal voltage loop to pinpoint the break.
The compressor starts but shuts down after a few seconds. Why?
This often points to IPM failure, low DC bus voltage, or compressor lockup. Our workflow helps you test the IPM diode drops and compressor windings.
Do I need special tools for mini-split diagnostics?
A good multimeter with True RMS and a clamp are essential. An oscilloscope is helpful but not required if you follow our voltage-based communication checks.
Know What to Check Next on Your Next Call
Get the checks in order, confirm the cause, and fix it right the first time.