AutoCodes – Mercury – P0491 Mercury

P0491 Mercury Code - Secondary Air Injection System Insufficient Flow Bank 1

Quick Answer

An air pump is used on the vehicle to lower tail pipe emissions on start-up. Common causes include Secondary air pump with no or low air flow, Secondary air bypass solenoid leaking /blocked or stuck open /closed, Secondary AIR air hose restricted. Severity is Low - monitor the vehicle and repair when practical. The vehicle can usually be driven if no severe symptoms are present, but symptoms should be checked before extended driving.

What it means: P0491 Mercury indicates Secondary Air Injection System Insufficient Flow Bank 1.
Common causes: Common causes include Secondary air pump with no or low air flow, Secondary air bypass solenoid leaking /blocked or stuck open /closed, Secondary AIR air hose restricted.
Severity: Low - monitor the vehicle and repair when practical.
Can I drive?: The vehicle can usually be driven if no severe symptoms are present, but symptoms should be checked before extended driving.

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Code Popularity 1,235

How popular is this code, among other codes? Low

Repair Importance Level

Low (Monitor) (1 / 3)

Repair Difficulty Level

Intermediate (2 / 3)

What is this?

Table of contents Created and updated by William Orellana on Aug 13, 2020

Description
Possible Causes
How To Fix
Tech Notes ▶
Cost to Diagnose
FAQs
Comments
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Code P0491 Mercury Description

An air pump is used on the vehicle to lower tail pipe emissions on start-up. The Powertrain Control Module (PCM) grounds the Air pump relay control circuit which energizes the Air pump. The PCM also grounds the Air combination valve vacuum control solenoid circuit, which energizes the Air vacuum control solenoid. Vacuum is then applied to both Air combination valve diaphragms which opens the shut off valves. The PCM enables both circuits simultaneously when Air system operations id desired. When the AIR system is active, then Air pump forces fresh air into the exhaust stream in order to accelerate catalyst operation. The AIR combination valves replace the conventional check valves. When the AIR system is inactive the shut off valves prevent air flow in either direction.

OBDII Code P0491 Mercury - Secondary Air Injection System Insufficient Flow Bank 1 - AutoCodes.com — P0491 Mercury Code - Secondary Air Injection System Insufficient Flow Bank 1

What does this mean?

What are the Possible Causes of the DTC P0491 Mercury?

Secondary air pump with no or low air flow
Secondary air bypass solenoid leaking /blocked or stuck open /closed
Secondary AIR air hose restricted
Secondary air vacuum hoses restricted or leaking

How to Fix the DTC P0491 Mercury?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What does this mean?

Code Tech Notes

User Submitted Video

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0491 Mercury code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

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About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0491 Mercury Code

What are common symptoms of OBDII code P0491 in a Mercury related to the Secondary Air Injection System?

Common symptoms include the Check Engine Light illuminating, rough idling, decreased fuel efficiency, and potentially a noticeable decrease in engine performance.

What are some possible causes of OBDII code P0491 in a Mercury?

Possible causes include a faulty air pump, a malfunctioning air pump relay, clogged air injection lines, or a failed air pump check valve.

How can I diagnose the OBDII code P0491 in my Mercury?

Diagnosis typically involves scanning the vehicle's computer for the specific code, inspecting the air injection system components for damage or blockages, and testing the functionality of the air pump and related components.

What are some repair tips for resolving OBDII code P0491 in a Mercury?

Repair tips may include replacing the faulty air pump, clearing any obstructions in the air injection lines, checking and replacing the air pump relay if needed, and ensuring proper operation of the air pump check valve.

Is it safe to continue driving my Mercury with OBDII code P0491 present?

It is generally not recommended to continue driving with this code present, as it can lead to further damage to the engine and emission control system. It is best to address the issue promptly to prevent potential issues.

Can a DIY enthusiast fix OBDII code P0491 in a Mercury?

Depending on the individual's skill level and access to tools, some DIY enthusiasts may be able to address this code by following repair guides and utilizing the necessary parts. However, professional assistance is recommended for those less experienced.

Are there any recalls or technical service bulletins related to OBDII code P0491 in Mercury vehicles?

It is advisable to check with the manufacturer or authorized dealerships to inquire about any recalls or technical service bulletins specific to OBDII code P0491 in Mercury vehicles to see if there are any related issues.

Is the repair for OBDII code P0491 in a Mercury covered by any warranties?

If the vehicle is still under warranty, the repair for OBDII code P0491 may be covered, depending on the terms and conditions of the warranty. It is recommended to consult the warranty documentation or contact the dealership for clarification.

Can OBDII code P0491 in a Mercury trigger other related codes?

In some cases, OBDII code P0491 may trigger additional codes related to the secondary air injection system or other connected components. It is essential to diagnose and address all relevant codes to ensure proper functioning of the vehicle.