What is the Secondary Air Injection System?

Secondary Air Injection System Illustration

The Secondary Air Injection System (SAIS) is a part of the vehicles exhaust system. Manufacturers began introducing them in the 1960’s to help reduce hydrocarbon and carbon monoxide emissions, both of which are result of incomplete combustion. On Toyota and Lexus vehicles that have SAIS it is only commanded to operate after a cold start, which is when the engine has been sitting for 7 hours or longer. The SAIS provides oxygen rich ambient air into the exhaust manifold downstream of exhaust valves. It will do this for approximately 30 – 90 seconds until the engine gets to operating temperature and then shut off.

What is the purpose of the Secondary Air Injection System?

It is no secret that gasoline engines are not 100% efficient. As a result, there is portion of unburned fuel (hydrocarbons) that does not get converted to engine power. This is particularly true on cold starts for two reasons. First, for more reliable starting the fuel/air mixture is going richer in fuel. Second, it takes a couple of minutes for the engine and catalytic convertors to get up to operating temperatures. In combination, these two factors increase amount of unburned fuel after a cold start. Thus, the purpose of the Secondary Air Injection System; to reduce increased emissions of hydrocarbons and carbon monoxide that is present due to cold start conditions.

What components make up the Secondary Air Injection System?

There are several components that make up the Secondary Air Injection System. This includes; Air Injection Drivers, Air Pumps, Air Switching Valves and Pressure Sensors. The location of these components will vary depending on year, model, and engine size. Toyota and Lexus vehicles can have 1 Bank or 2 Bank SAIS configurations. This determines the number of Air Injection Drivers and Air Pumps that are a part of the SAIS.

How does the Secondary Air Injection System Work?

Again, the purpose of the Secondary Air Injection System is to reduce emissions after a cold start by burning fuel that made it through the engine un-combusted. But how is this accomplished? As we learned in high school science class, oxygen is one of the elements required for combustion. Most of the air sent through the induction chamber for primary combustion is depleted, leaving exhaust lacking oxygen. That said, the SAIS reintroduces oxygen rich air into exhaust allowing unburned fuel to continue burning before being expelled into the atmosphere.

This process is carried out by the various components of the SIAS as follows. On cold start ignitions the Air Injection Drivers will command the Air Switching Valves to open and the Air Pumps to, you guessed it, pump air. The air is sent through the Air Switching Valves and injected into exhaust manifold up stream of the catalytic converters. When the air/oxygen provided by the SAIS mixes with the post combustion exhaust oxidation occurs. Now replenished with oxygen hydrocarbons and carbon monoxide can oxidize into less harmful emissions like water and carbon dioxide. The continued burning of fuel also increases the temperature of exhaust gas which helps the catalytic converters heat up to operating temperature quicker. It typically takes 30 -90 seconds and once operating temperature is achieved air from the SAIS is no longer needed and the SAIS stops operating. The Air Pumps turns off and the Air Switching Valves close. If the Air Switching Valve does not close than exhaust can leak back into the SAIS which can lead to additional issues.

How does it affect the vehicle if the Secondary Air Injection System is not working properly?

Mechanically speaking since the Secondary Air Injection System only effects post combustion operations; a nonfunctioning SAIS does not impact engine performance or fuel economy. Therefore, the sole function of the SIAS is emission reduction. That said, the SAIS will only run on a cold start when the vehicle has been sitting for 7 hours or longer. Even then, it will only run for 30 – 90 seconds until the operating temperature has been reached. So it can be more accurately stated that, the SAIS reduces emissions at most once every 7 hours for 30 – 90 seconds. Outside of that specific time period the SAIS is inactive. Consequently if Secondary Air Injection System is not working properly then exhaust will be richer after a cold start until operating temperature is reached. Unfortunately, there are certain Secondary Air Injection System error codes that can put the vehicle into “Limp Home Mode”. This is a reduced performance mode where power and acceleration can be limited.

Hewitt Technologies Secondary Air Injection Bypass Kit Solutions

To learn more about our different Bypass Kits and which one is best for you, read our Blog post Secondary Air Bypass Kit Comparison

What do my Secondary Air Injections Error Codes Mean?

To learn more about the different types of SAIS fault codes and more details about specific error codes, read our Blog post What Do My Error Codes Mean?

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1 Comment

  1. José v on November 22, 2021 at 9:32 pm

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