What does the secondary air pump do on a Subaru?
The secondary air pump in a Subaru injects fresh air into the exhaust during cold starts to help the catalytic converter heat up and reduce emissions. It’s part of Subaru’s Secondary Air Injection (SAI) system that’s used on many modern models.
How it works
In Subaru vehicles, the secondary air pump is typically an electric motor driven unit located in the engine bay. It draws in ambient air and pushes it through a short, dedicated path into the exhaust system—usually near the exhaust manifold or into the exhaust stream just after the manifold—via a valve assembly. The engine control module (ECM) activates the pump during cold starts or when the engine is warming up, ensuring that air mixes with the exhaust to promote oxidation of unburned fuel and reduce emissions.
Key components involved
Understanding the system helps explain its function:
Here are the main functions of Subaru's secondary air pump:
- Reduces cold-start emissions by oxidizing hydrocarbons (HC) and carbon monoxide (CO) in the exhaust.
- Speeds up catalytic converter light-off, helping the converter reach its operating temperature faster.
- Supports overall emissions performance during warm-up to meet regulatory standards.
- Works as part of the broader air-injection system to improve exhaust after-treatment efficiency.
These elements work together to lower pollutants released during the initial moments after starting the engine and during a cold engine. By accelerating the catalyst’s warm-up, the SAP helps shorten the window of higher emissions after ignition.
When it runs and why it matters
The SAP typically activates for a short period after engine start or when the engine is cold, and it may shut off once the catalytic converter reaches its operating temperature. In some models, the system may operate again under certain driving conditions to maintain optimal emissions performance. The activation is controlled by the PCM/ECM based on sensor inputs, engine temperature, and startup status.
Diagnosing and maintaining the SAP
Like any emissions component, the secondary air system can develop faults. Diagnosis usually involves scanning for specific trouble codes and inspecting the pump, valves, hoses, and electrical wiring. In Subaru vehicles, common diagnostic codes related to this system include P0410 (Secondary Air Injection System Malfunction) and related variants that indicate a problem with the air delivery path or the pump itself.
Here are common symptoms that may indicate a failing SAP or related components:
- Check Engine Light (CEL) illuminated with P0410 or related codes.
- Unusual noise coming from the engine bay, often from a failing pump or belt-driven unit.
- Poor cold-start performance with higher HC/CO emissions noted on tests or by odor.
- Reduced effectiveness of the exhaust after-treatment during startup or warm-up.
- Visible leaks, damaged hoses, or wiring issues around the SAP assembly.
If you notice these symptoms, have the vehicle inspected by a qualified technician. Diagnosis typically includes checking the pump operation, valve function, air lines, check valves, and related electrical circuits. Replacement or repair should address the root cause to restore proper emissions performance.
Maintenance considerations
Regular inspection of the SAP system during routine maintenance can prevent failures from escalating. If the system has been serviced recently, ensure the correct hoses and clamps are reinstalled and that electrical connectors are secure. If a fault is diagnosed, replacing the pump, associated valves, or corroded wiring is common, and programming or reset of the ECM may be required after repair.
Summary
In short, Subaru’s secondary air pump helps curb emissions by delivering fresh air to the exhaust during cold starts, accelerating catalytic converter warm-up, and supporting the overall efficiency of the emissions control system. A functioning SAP reduces pollutants early in engine operation, while a faulty unit can trigger diagnostic codes and higher emissions. If problems arise, professional diagnosis and appropriate repair are recommended to restore optimal performance and compliance with emissions standards.
