P0491 Code: Meaning, Symptoms, Causes & Fix

Overview

P0491 is triggered when monitoring logic identifies an issue with Secondary Air Injection System (Bank 2) affecting powertrain controls.

Symptoms

increased fuel use
reduced performance
rough operation in certain conditions

Common causes

Control module logic or calibration issue affecting secondary air injection system (bank 2)
Component failure in the monitored system
Connector pin damage/corrosion or harness stress near related components

How to diagnose P0491

Confirm P0491 and capture freeze-frame data with a professional scan tool.
Inspect wiring, connectors, and grounds related to secondary air injection system (bank 2) for obvious defects.
Use freeze-frame and live data to isolate whether the issue is electrical, mechanical, or control logic.
Check for related DTCs to identify the root-cause chain before replacing parts.
After repair, clear codes and confirm readiness monitors complete without recurrence.

Severity of code P0491

Low: This code is low to moderate severity in most cases, but diagnosis is still recommended to avoid recurring warnings.

Can you drive with P0491?

The vehicle is often drivable with P0491, but plan a repair window to prevent repeat faults.

If the warning lamp is flashing or drivability/safety is affected, avoid driving and diagnose immediately.

How to fix it (step-by-step)

Repair obvious wiring/connector faults first to prevent repeat parts replacement.
Address the root fault mode (generic) in the secondary air injection system (bank 2) circuit/system.
Apply the required relearn/adaptation procedure if specified by service information.
Road-test and verify P0491 remains cleared under the conditions that originally set the code.

Fix options

Test component operation and replace if faulty.
Inspect circuit and connector condition.
Clear DTCs and verify with live data and completed drive cycle.

Typical repair cost

$100-$450

Common mistakes when diagnosing P0491

Replacing parts without verifying voltage, ground, and signal integrity first.
Ignoring related stored/pending codes that indicate upstream faults.
Skipping post-repair verification drive cycle and readiness checks.

P0491 in common vehicles

P0491 appears across makes such as Mazda, Subaru, Lexus, Jeep. Failure patterns differ by platform: wiring routes, software calibration, and component supplier revisions can change root cause.

Frequently asked questions

What causes P0491?

P0491 is commonly set by generic faults affecting secondary air injection system (bank 2).

How much does it cost to fix P0491?

For P0491, repair costs are typically $100-$450 depending on exact root cause, labor rates, and part quality.

Can I clear P0491 without repair?

You can clear P0491 temporarily, but the code usually returns until the underlying fault is corrected.