P04xx Codes: Auxiliary Emissions Controls
This family of codes deals with the various systems your car uses to keep its exhaust clean.
Overview of P04xx Codes
The P04xx codes are a broad category covering secondary emissions control systems.
Common P04xx Codes:
- P0420 & P0430: The most famous of the group, indicating catalyst system inefficiency for Bank 1 and Bank 2, respectively.
- P0401: EGR (Exhaust Gas Recirculation) Flow Insufficient. This means the EGR system is clogged or failing, which can cause engine knocking.
- P0442: Evaporative Emission System Leak Detected (Small Leak). This is often caused by a loose or faulty gas cap.