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When NOx remains high despite verified EGR operation, the catalytic converter becomes the suspect. Catalysts play a secondary role in reducing NOx after combustion. If the efficiency monitor fails, it strongly suggests the catalyst is no longer functioning effectively. A MAP sensor or vacuum leak would primarily affect mixture trims, not NOx alone. Faulty injectors can create imbalance but do not explain catalyst efficiency codes. ASE stresses the dual roles of EGR and catalysts in NOx control. When EGR is functional, persistent NOx elevation nearly always points to catalyst deterioration.

If the downstream O2 sensor mirrors the upstream sensor, the catalyst is not storing oxygen, indicating failure. O2 sensor faults may cause flat signals, but mirroring patterns confirm catalyst inefficiency. Exhaust leaks or vacuum leaks would not cause this O2 pattern. ASE stresses comparing sensor activity for catalyst evaluation.