What does the secondary air system do?

The secondary air system injects fresh air into the exhaust to help burn off unburned fuel and heat up the catalytic converter, reducing emissions during cold starts.

In practice, the system is activated by the engine management computer and uses a pump, valves, and ducts to route air into the exhaust at specific times. Its presence and intensity vary by vehicle, with newer designs sometimes relying less on secondary air as catalysts reach operating temperature more quickly and as aftertreatment technology evolves.

How the system works

These are the core steps and components that make secondary air injection function in most gasoline engines.

• Air source: a small electric pump or belt-driven pump provides fresh, ambient air.


• Air delivery path: ducts or pipes route air to the exhaust manifold or directly into the exhaust stream ahead of the catalytic converter.


• Control valves and solenoids: valves regulate when and how much air is released, managed by the engine control unit (ECU) based on temperature and engine speed.


• Exhaust-side integration: air mixes with exhaust gases to promote oxidation of hydrocarbons (HC) and carbon monoxide (CO) and to help warm up the catalytic converter.


• Sensors and feedback: oxygen sensors, temperature sensors, and sometimes pressure sensors monitor performance and adjust operation.


• ECU coordination: the system is typically activated during cold-starts or specific operating conditions to meet emission targets.

In practice, the system runs briefly during cold starts or high-emission conditions, then reduces or stops when the catalytic converter reaches its effective temperature.

Why it matters

Secondary air injection helps reduce exhaust pollutants at a critical phase of engine operation. By boosting oxidation in the exhaust, it lowers emissions of hydrocarbons and carbon monoxide and can shorten catalytic converter warm-up time, enabling faster compliance with emissions standards.

• Emission reduction during cold start: oxidizes HC and CO before the catalyst becomes fully effective.


• Catalytic converter warm-up: speeds up reaching operating temperature for peak efficiency.


• NOx considerations: SAS mainly targets HC and CO; catalytic efficiency improvements indirectly support NOx control, but NOx is primarily managed by other aftertreatment systems such as EGR and SCR.


• Energy source: typically uses a small electric motor or belt-driven pump, with minimal impact on overall fuel economy.

Overall, SAS is part of the aftertreatment strategy that helps vehicles meet regulatory limits while maintaining driveability during warm-up.

Maintenance and troubleshooting

Like any aftertreatment system, the secondary air system can develop faults that trigger diagnostics. Common issues include a failed air pump, clogged or leaking ducts, malfunctioning valves, or faulty sensors. Symptoms may include a check engine light, rough idle, or failed emissions tests. Regular maintenance and professional diagnosis are advised if a fault is suspected.

Differences across powertrains

Gasoline engines versus diesels

In traditional gasoline engines, secondary air injection is most active during cold starts to help catalytic converters reach temperature quickly. In some diesel applications, secondary air or related oxidation strategies may be used in conjunction with exhaust aftertreatment to reduce certain pollutants, but diesels often rely on particulate filters and selective catalytic reduction (SCR) for NOx control, with SAS being less common or differently configured.

Electric versus belt-driven sources

Newer vehicles may use electric air pumps instead of belt-driven or pulsed air systems, enabling finer control and independence from engine speed, which improves efficiency and reduces parasitic losses when idle or at low RPM.

Coordination with other systems

SAS interacts with other emissions controls, including exhaust gas recirculation (EGR), catalytic converters, and aftertreatment controllers. In modern cars, the ECU coordinates multiple subsystems to minimize emissions across drivability scenarios.

Summary

The secondary air system is a targeted emissions-control feature that injects fresh air into the exhaust to oxidize unburned fuel and heat the catalytic converter during cold starts. Its design varies by vehicle, but the core purpose remains to reduce pollutants at the moment when the exhaust is farthest from evolving fully efficient emissions control. By using a pump, valves, ducts, and sensors, the SAS works in concert with other aftertreatment systems to help engines meet modern environmental standards while preserving performance and driveability.

Can a car run without secondary air injection?

Which begs the question: Does a car need a secondary air injection system to run? Strictly speaking, no, it doesn't. If your car doesn't have a smog pump, a couple things can happen.

What happens when the secondary air pump goes bad?

When a secondary air pump goes bad, it can cause the check engine light to illuminate, lead to increased emissions, and result in drivability issues like hesitation, rough idle, or stalling. Other potential symptoms include decreased fuel economy, a reduction in overall power, and a failed emissions test. In some cases, a failing pump can also produce noises or a burning smell.
 
This video explains what a secondary air injection system is and how it works: 18sDeutsche Auto PartsYouTube · Mar 7, 2016
Symptoms of a failed secondary air pump

• Check Engine Light: This is a very common indicator that the secondary air injection system is not working correctly. You may also see specific trouble codes, such as P0410 or P0411, according to Bhadra Electronics. 
• Performance issues: The vehicle may hesitate, stumble, or feel a lack of power when accelerating. Some drivers experience a rough idle or even stalling. 
• Increased emissions: The pump is designed to help reduce harmful emissions during a cold start. Its failure can cause the vehicle to fail emissions tests. 
• Decreased fuel economy: The engine may run richer for longer during cold starts, using more fuel than normal. 
• Audible or visible signs: A malfunctioning pump can cause rattling or other unusual sounds. In some cases, a stuck pump relay can overheat and cause a burning smell or melting on the electrical components. 
• Difficulty starting: In some vehicles, a bad pump can make it more difficult to start the engine. 

How much does it cost to repair a secondary air injection system?

Fixing a secondary air injection system can cost anywhere from $400 to over $1200, with the price depending on the specific issue. A full replacement of the air pump is typically the most expensive repair, potentially costing up to $950-$1200, while other repairs like fixing vacuum hoses or replacing a relay are much cheaper.
 
Repair cost breakdown

• Air injection pump replacement: The most costly repair, often ranging from $250 to over $950, depending on the vehicle model and labor rates.
• Vacuum hoses: These are a very inexpensive fix, costing only about $20 to $50 to replace.
• Other repairs: Fixing other components like the combination valve, a faulty relay, or a vacuum leak can fall in the $350 to $500 range. 

What determines the cost?

• The specific faulty part: Replacing a simple hose is far cheaper than replacing a complex air pump.
• Labor time: Some repairs require more labor than others. For example, the air pump is often more difficult to access and replace. 
• Vehicle model: The cost of parts and labor can vary significantly between different makes and models. 
• Dealership vs. independent mechanic: Dealerships may have higher labor rates, but independent mechanics can sometimes be more affordable. 

What you can do

• Check your warranty: Some emissions systems are covered by a separate warranty. 
• Look for recalls: Some vehicles have recalls for the secondary air injection system, which can make the repair free. 
• Inspect vacuum lines: Before taking it to a mechanic, you can visually inspect the vacuum lines for any obvious cracks or disconnections. 

Note: The final cost will vary depending on the specific diagnosis and repair needed for your vehicle. It is best to get an estimate from a qualified mechanic for an accurate price.

What does a secondary air system do?

That are the most important parts of the secondary. Air system the main one is the obviously secondary air pump which is an electric pump. And actually pumps the air into the exhaust.