⚠️ Before You Start
- Pre-condition: LOCH code is active (fan should start but is not running).
- Assumption: Controller/thermostat controls are functioning properly.
🛠️ Phase 1: Disable Lock Rotor Protection (LRPT)
You must disable LRPT first to prevent the system from cutting power during testing.
| Step | Action | Expected Display / Result |
|---|---|---|
| 1 | Turn off the fan command from the thermostat/controller & cycle power. | System resets |
| 2 | Press and hold the Set button for 3 seconds. | Hi1 |
| 3 | Press the Minus (-) button 4 times. | LrPt |
| 4 | Press the Set button once. | on |
| 5 | Press the Minus (-) button once. | off |
| 6 | Press and hold the Set button for 3 seconds to save. | Settings locked |
| 7 | Turn the fan command back ON. | System ready for testing |
⚡ Phase 2: Signal & Voltage Diagnostics
With the fan commanded ON and LRPT OFF, measure voltages at the 12-pin plug (Motor plug or Drive plug for $\ge$ 1.5 HP).
- Note: AC Ground is P9. DC Ground is P3.
| Test Point | Target Reading | If Reading is Correct | If Reading is Incorrect / Action |
|---|---|---|---|
| AC Line Voltage (P1 - P7 - P12) | Line Voltage (P12 is 3-phase only) | AC power supply is good. Proceed to next step. | Check main power source or supply wiring. |
| DC PWM Control Signal (P3 to P4) | 4.5 VDC | Control signal is good. Proceed to Phase 3 to isolate Motor vs. Drive. | Go to Phase 4 (Wiring/Board Issues). |
⚙️ Phase 3: Isolating Motor vs. Drive (For units $\ge$ 1.5 HP)
If you verified 4.5 VDC at P3/P4 but the motor still won't run, use this table to isolate the failure.
| System Type | Test Location & Action | Result | Diagnostic / Action |
|---|---|---|---|
| Without Drive | 12-pin plug (P3 to P4) | 4.5 VDC | ❌ Bad Motor (Replace motor) |
| With Drive | 12-pin plug (P3 to P4) | 4.5 VDC | Proceed to test Drive Output below |
| Drive Output | Yellow (or red) plug from drive to motor. Measure each of the (3) hot wires against green ground. | 150 - 250 VAC | ❌ Bad Motor (Replace motor) |
| Drive Output | Same as above. | < 150 VAC | ❌ Bad Drive (Replace drive) |
🔌 Phase 4: Wiring & Board Diagnostics
If you measured 0 VDC at P3/P4 in Phase 2, use this table to pinpoint the control failure based on your board type.
| Control Board | Action | Result | Diagnostic / Action |
|---|---|---|---|
| Symbio / UC400 | Test continuity on blue board-to-motor/drive wiring. | Continuity | ❌ Replace ECM Board |
| Symbio / UC400 | Same as above. | No Continuity | 🛠️ Repair / replace wiring harness |
| CSTI (Red Board) | Unplug J4 and motor harness. Test continuity between J4-1 & J4-2 and P3 & P4. | Continuity | ❌ Replace ECM & Red Board |
| CSTI (Red Board) | Same as above. | No Continuity | 🛠️ Repair wiring harness |
🔍 RPM Feedback Diagnostics (LOCH / Low Airflow active, but fan runs)
If the fan actually runs when LRPT is disabled or in commission mode, but still triggers a fault, the RPM feedback loop is failing (MtR1 reads 0).
| Control System | Check Point | Action |
|---|---|---|
| All Systems | Pin P10 | Verify wire is secure and not loose at the 12-pin plug. |
| CSTI | Trace wire from J4-3 to P10 | Secure connections or replace harness if broken. |
| Symbio / UC400 | Trace wire from J2-10 to P10 | Secure connections or replace harness if broken. |
- Key Takeaway: Always turn LRPT OFF before testing. If you have 4.5 VDC at P3/P4, the issue is downstream (Drive or Motor). If you have 0 VDC, the issue is upstream (Wiring or Board).