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SPC Troubleshooting
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| Symptom | Cause | Fix |
|---|---|---|
| "No data points yet. Waiting for trigger events..." | Trigger tag has not changed; tags not subscribed; device offline | Check the trigger tag changes on the device page. Both tags must be subscribed on the group's device. |
| Tag dropdowns are empty | No subscribed tags on the group's device | Subscribe the tags first. |
| Two samples per part | Trigger changes to two different non-zero values per part (e.g. 1 then 2 before resetting) | Pulse a single value, or trigger on a counter. |
| No samples from a "part done" bit | The bit stays at 1 between parts, so the value doesn't change | Have the PLC drop the bit back to 0 after each part. |
| Samples stop while another part runs | Group has a Program name that differs from the device's program tag | Expected. Create a group per program. |
| Samples are one part behind | Measurement and trigger update in the same poll | Update the measurement before the trigger in the PLC. |
| Some samples missing | Measurement value was non-numeric at trigger time | Check the tag's data type and quality. |
| Older samples no longer on the plot | The plot shows the latest 200 samples | Read older samples with GET /api/spc/charts/:chartId/data?to=…. |
| UCL/LCL never appear | Fewer than 25 samples, or all samples identical (σ = 0) | Wait for the baseline to fill, or enter the limits. |
| UCL/LCL do not follow the latest data | By design: limits are frozen once established | Expected. After a real process change, use Recalculate the control limits. |
| Limits look wrong after a machine rebuild | They still describe the process before the change | Recalculate them, or enter limits from a fresh capability study. |
| A limit says "set by an earlier version" | An earlier release saved automatic limits where typed-in ones go, so they cannot be told apart | Recalculate, or clear with PUT /api/spc/charts/:id {"ucl": null, "lcl": null} to re-baseline. |
| "Need 25 samples to recalculate control limits" | The chart has fewer samples than the recalculation window | Wait for more samples. |
| Cp shows "—" | Only one spec limit, fewer than 2 samples, or σ = 0 | Set both USL and LSL. |
| Cpk shows "—" | No spec limits | Set USL and/or LSL. |
| No shield icon, Mean line or Cpk trend | Enhanced SPC is off | Admin: System > Settings → Feature Toggles. |
| 403 Insufficient permissions adding a manual value | Manual entry needs editor | Use an editor account. |
| 403 deleting a group | Deleting groups needs admin | Ask an admin. |
| "chartName, readVariable, and triggerVariable are required" | API call missing fields | Supply all three. |
| "SPC chart not found" / "SPC group not found" | Wrong id or already deleted | List groups and charts to get ids. |