diff --git a/svc/product_series_auto_scheduler.go b/svc/product_series_auto_scheduler.go
index 80503ac..01b7a2e 100644
--- a/svc/product_series_auto_scheduler.go
+++ b/svc/product_series_auto_scheduler.go
@@ -101,6 +101,14 @@ func productSeriesFallbackAutoCreateEnabled() bool {
 	return raw == "1" || raw == "true" || raw == "on" || raw == "yes"
 }
 
+func productSeriesFallbackLogEnabled() bool {
+	raw := strings.TrimSpace(strings.ToLower(os.Getenv("PRODUCT_SERIES_FALLBACK_LOG")))
+	if raw == "" {
+		return false
+	}
+	return raw == "1" || raw == "true" || raw == "on" || raw == "yes"
+}
+
 func productSeriesFallbackCode() string {
 	code := strings.TrimSpace(os.Getenv("PRODUCT_SERIES_FALLBACK_SERIES_CODE"))
 	if code == "" {
@@ -485,11 +493,14 @@ func productSeriesApplyVariant(ctx context.Context, pg *sql.DB, v productSeriesA
 		return 0, 1, err
 	}
 	if !ready {
+		if productSeriesFallbackLogEnabled() {
+			log.Printf("[ProductSeriesFallback] skip_not_ready product=%s color=%s dim3=%s", strings.TrimSpace(v.ProductCode), strings.TrimSpace(v.ColorCode), strings.TrimSpace(v.Dim3Code))
+		}
 		return 0, 1, nil
 	}
 	selected := productSeriesSelectRules(v, rules)
 	if len(selected) == 0 {
-		fallbackID, _, err := productSeriesResolveFallbackSeries(ctx, pg)
+		fallbackID, fallbackCode, err := productSeriesResolveFallbackSeries(ctx, pg)
 		if err != nil {
 			return 0, 1, err
 		}
@@ -505,15 +516,24 @@ WHERE mmitem_id=$1
 LIMIT 1
 `, mmitemID, dim1ID, nullableInt64ForAuto(dim3ID)).Scan(&exists)
 			if checkErr == nil {
+				if productSeriesFallbackLogEnabled() {
+					log.Printf("[ProductSeriesFallback] already_exists product=%s color=%s dim3=%s fallback=%s(%d)", strings.TrimSpace(v.ProductCode), strings.TrimSpace(v.ColorCode), strings.TrimSpace(v.Dim3Code), strings.TrimSpace(fallbackCode), fallbackID)
+				}
 				// keep existing manual/previous assignment; nothing to do
 				return 0, 0, nil
 			}
 			if checkErr != nil && checkErr != sql.ErrNoRows {
 				return 0, 1, checkErr
 			}
+			if productSeriesFallbackLogEnabled() {
+				log.Printf("[ProductSeriesFallback] apply product=%s color=%s dim3=%s fallback=%s(%d)", strings.TrimSpace(v.ProductCode), strings.TrimSpace(v.ColorCode), strings.TrimSpace(v.Dim3Code), strings.TrimSpace(fallbackCode), fallbackID)
+			}
 			// Use the fallback series as the single selected rule.
 			selected = []productSeriesAutoRule{{SeriesID: fallbackID}}
 		} else {
+			if productSeriesFallbackLogEnabled() {
+				log.Printf("[ProductSeriesFallback] missing_fallback product=%s color=%s dim3=%s fallback_code=%s", strings.TrimSpace(v.ProductCode), strings.TrimSpace(v.ColorCode), strings.TrimSpace(v.Dim3Code), strings.TrimSpace(fallbackCode))
+			}
 			return 0, 1, nil
 		}
 	}