Tell Bad Wiring From a Bad Board
Check the communication signal across the interconnect terminals (e.g. S1/S2/S3) to see which side has failed.
System Diagnostics
Know Whether It's the Wiring, the Board or the Compressor Before You Order an Inverter Part
From communication lockouts to a compressor that won't ramp up, ACLogics tells you what to check next, low-voltage first, until the cause is confirmed.
FAULT SYMPTOM
Inverter compressor will not start, LED shows a compressor sync loss or drive fault code
CHECK 01
Disconnect compressor plugs. Measure winding resistance across all 3 phases (U, V, W). Are windings balanced and isolated?
Yes, balanced under 2Ω, no ground path
CHECK 02
Power unit on. Measure the DC Bus voltage across the capacitor bank. Is voltage around 300-340VDC?
Yes, voltage is stableNo, voltage is extremely low or 0V
No, open windings or short to ground
DIAGNOSIS
Failed compressor motor windings (shorted or grounded). Replace compressor.
Complete diagnostic tree — every branch, every fault path
Try ACLogics Free — iPhone & AndroidHow a Call Goes
01
Enter the Code or Symptom
The blink code, thermostat error, or what it's doing, such as not ramping up.
02
Measure in a Safe Order
Go from low-voltage communication checks to high-voltage inverter checks to find where it fails.
03
Confirm Before You Order
Record the readings that show the cause before you order an expensive inverter part.
What You'll Stop Guessing About
Know what failed before you order the part.
Stop Replacing Boards That Weren't Broken
Check expansion valve coils and sensor readings first, so a metering or sensor fault isn't blamed on the board.
Catch a Locked Rotor Safely
Measure the power path step by step to tell a failed board from a locked compressor.
Frequently Asked Questions
How do I distinguish between a faulty inverter board and a bad compressor?
Measure the resistance across all compressor terminals (should be identical and balanced, typically under 2 ohms) and verify no continuity to ground. If the compressor is fine, check if the inverter module outputs balanced AC voltage to the compressor pins briefly during startup.
What is the correct way to test variable-speed communication (e.g., S1, S2, S3) issues?
Disconnect S3 and measure DC voltage between S2 and S3 on the outdoor board. It should pulse dynamically between 0V and 24VDC. A flat or static voltage indicates a communication circuit failure on one of the boards.
How do I verify if an Electronic Expansion Valve (EEV) is physically stuck?
Measure the resistance of the EEV actuator coil windings (usually 40-50 ohms). If the coil is good, use a strong magnet or manual EEV driver to listen for mechanical movement or monitor the change in refrigerant pressures.
What does a DC Bus Undervoltage fault code typically point to?
It indicates that the DC voltage on the main capacitors has dropped below the threshold (typically around 200VDC). Check for low supply voltage, a faulty rectifying bridge, a failed reactor, or heavy compressor load pulling down the bus.
Know What to Check Next on Your Next Call
Get the checks in order, confirm the cause, and fix it right the first time.