P0491

The P0491 code indicates the PCM has identified a secondary air injection system insufficient flow (bank 1) issue. This code is associated with the vehicle's emission control systems designed to reduce harmful exhaust pollutants. A malfunction in these systems can increase vehicle emissions beyond acceptable limits and will likely cause a failed emissions inspection. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.

Here are the key things to understand: Unusual exhaust smell. Failed emissions test. Reduced engine performance. Hissing or sucking sound from engine bay. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: Faulty air injection switching valve. Damaged or disconnected air injection hoses. Failed relay in the air injection system. Failed check valve in air injection system. Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: Connect an OBD-II scanner and read all stored trouble codes and freeze frame data. Inspect the secondary air injection pump for proper operation. Check all hoses and pipes in the air injection system for damage or leaks. Test the switching valve solenoid and relay with a multimeter. Verify airflow through the system using a flow meter if available. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

This article is part of our Powertrain (P) collection on OBD Codes. We provide comprehensive, up-to-date information to help you find solutions quickly.