What does a secondary air injection system do?
The secondary air injection system reduces emissions during cold starts by sending fresh air into the exhaust to promote the oxidation of unburned fuel, hydrocarbons and carbon monoxide, and to help the catalytic converter heat up faster.
In more detail, it uses an air pump and a series of valves to route air into the exhaust manifold or directly into the exhaust stream when the engine is cold or the converter is not yet at operating temperature. The system is controlled by the vehicle's ECU/PCM and is designed to complement the catalytic converter's performance, though many modern engines rely on other technologies and some models have phased it out.
How it works
The system boosts the amount of oxygen available in the exhaust to help burn off pollutants early in the exhaust stream, particularly during the critical startup period.
Before listing the parts involved, it helps to understand the core components that make the system function.
- Electric or belt-driven air pump
- Check valves and a diverter valve to control airflow direction
- Hoses or tubes that route air from the pump to the exhaust manifold or downstream ports
- Air control valve or solenoid to regulate when air is injected
- Engine Control Unit (ECU/PCM) that times injection based on sensor data
- Air filter or silencer to reduce pump noise
Concluding note: Together, these components supply and time the introduction of fresh air into the exhaust to support oxidation reactions during startup and warm-up, when the catalytic converter is not yet fully effective.
Before listing how the system is activated, note the typical operating conditions that trigger secondary air injection.
- During cold starts and the initial engine warm-up period
- When the catalytic converter temperature is below its efficient operating range
- Under ECU control using signals from engine temperature, RPM, and oxygen sensors
- Air directed to upstream (manifold) or downstream (converter inlet) locations, depending on design
- Diagnostics and fault monitoring; the system may disable if a fault is detected
Concluding note: This operational behavior helps ensure pollutants are oxidized early in the exhaust process, improving startup emissions performance.
Benefits and limitations
Understanding the trade-offs helps explain why automakers use (or sometimes phase out) secondary air injection.
- Reduces hydrocarbon and carbon monoxide emissions during cold starts
- Speeds up catalytic converter warm-up, helping the exhaust system reach efficiency faster
- Can improve results on emissions tests that measure startup periods
- Limited impact on NOx emissions in many designs
- Adds additional components that can fail, increasing maintenance considerations
Concluding note: While helpful for startup emissions, SAI is typically just one part of a broader emissions strategy and may be absent on newer designs that rely on advanced catalysts, direct injection, and other technologies.
Maintenance and common issues
Like other emission-control systems, secondary air injection can experience wear or faults that trigger diagnostic warnings.
- Faulty or seized air pump, reducing or eliminating air delivery
- Leaks or cracks in hoses and fittings, causing loss of pressure or unintended airflow
- Sticking or failed check/diverter valves that misroute air
- Electrical problems: damaged wiring, blown fuses, or faulty solenoids
- Sensor or ECU faults that prevent proper timing or disable the system
- Excessive noise from the pump or lines indicating a mechanical issue
Concluding note: If the vehicle’s check engine light is on or you notice rough idle or poor startup emissions, a diagnostic scan and physical inspection of the pump, valves, and lines are a typical starting point.
Summary
The secondary air injection system is an emissions-control feature that injects fresh air into the exhaust during cold starts to promote the oxidation of remaining pollutants and to help the catalytic converter reach its operating temperature faster. It relies on a pump, valves, ducts, and ECU control to time the delivery of air, providing a useful emissions benefit during startup while adding potential maintenance considerations. As vehicle technology evolves, some models retain SAI as a cost-effective startup-emission solution, while others have moved toward alternative strategies.
What happens when a secondary air injection pump fails?
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.
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 are the symptoms of a bad secondary air injection valve?
Potential complaints:
- Secondary air pump does not start after a cold start.
- The engine compartment smells charred.
- Melting traces on the electrical contacts of the secondary air pump.
- P0410 “Malfunction”
What is the purpose of secondary air injection?
So secondary. Air is essentially an emissions control system this injects. Air into the exhaust on cold. Start. The function of this is to heat up the catalytic converters.
