AP Exhaust 644003 Catalytic Converter - Center

AP Exhaust Direct Fit Converter for Nissan Maxima

AP Exhaust Part Number 644003 is a Federal / EPA direct-fit catalytic converter built for select Nissan Maxima applications. It uses a stainless steel body, a ceramic substrate, and OE-style flange connections to match factory dimensions and fitment for center-position installations.

The AP Exhaust Direct Fit Converter line focuses on exact OE sizing and consistent manufacturing. For Nissan Maxima owners, that means a replacement converter engineered around factory-style fitment, with welded flanges, gasket provisions, and emissions-system compatibility for the listed applications.

Key Features

• Part number 644003
• Federal / EPA direct-fit catalytic converter
• Stainless steel body construction
• Ceramic substrate material
• 3-bolt welded flange inlet with gasket stub
• 2-bolt welded flange outlet
• Overall length: 41.5 in.
• O2 sensor ports: none
• Outlet gasket and outlet hardware included
• Compatible with center-position Nissan Maxima applications listed below

Detailed Description

AP Exhaust 644003 is a direct-fit catalytic converter for Nissan Maxima models with Federal and California emissions configurations in the listed years. It is built with stainless steel for corrosion resistance and a ceramic substrate for emissions conversion performance. The OE-style flange layout helps maintain factory-style alignment during replacement, while the exact-dimension manufacturing approach supports consistent fitment across the covered applications.

This part is listed for 2021-2023 Nissan Maxima, center; 2016-2020 Nissan Maxima, center; and 2009-2014 Nissan Maxima, center. It also appears in additional Nissan Maxima fitments for 2016-2017 and 2009-2014 with N/A position notes in the catalog data. AP Exhaust 644003 is a practical replacement choice when servicing the exhaust system on a Nissan Maxima and restoring proper catalytic converter function.

This part cross-references Walker 16337 and MagnaFlow 51356 for the same application.

What It Is Used For

• Replacing a worn, damaged, or restricted catalytic converter
• Restoring exhaust flow on a Nissan Maxima center exhaust section
• Addressing emissions-related fault codes tied to converter efficiency
• Replacing a converter with OE-style flange fitment
• Servicing Federal / EPA emissions applications listed for the vehicle

Installation Notes and Tips

Verify the vehicle year, emissions configuration, and position before ordering. This converter uses a 3-bolt welded flange with gasket stub at the inlet and a 2-bolt welded flange at the outlet. O2 sensor ports are not provided, so confirm the original exhaust layout matches the part before installation. Check local emissions regulations before use, especially for vehicles licensed in CA, CO, ME, and NY where restrictions are noted in the fitment data.

Quality Assurance

AP Exhaust states that its Direct Fit Converter line is manufactured to exact OE dimensions using automated workstations, spinner welding equipment, and high-speed production lines. The brand also uses proprietary substrate chemistry to support efficient catalyst loading and consistent converter performance across the covered applications.

• Fits 2021-2023 Nissan Maxima, center
• Fits 2016-2020 Nissan Maxima, center
• Fits 2009-2014 Nissan Maxima, center
• Fits 2016-2017 Nissan Maxima, N/A
• Fits 2009-2014 Nissan Maxima, N/A