What is the Secondary Air Injection System?

Secondary Air Injection System Illustration

The Secondary Air Injection System (SAIS) is part of a vehicle’s exhaust system. Manufacturers began introducing it in the 1960s to help reduce hydrocarbon and carbon monoxide emissions, both of which resulted from incomplete combustion. Toyota and Lexus SAIS systems are only commanded to operate after a cold start, which is when the engine has been sitting for at least seven hours.

The SAIS injects oxygen-rich ambient air into the exhaust manifold downstream of exhaust valves. It does this for approximately 30 to 90 seconds to allow the engine to get to operating temperature. The SAIS then shuts off.

What is the Purpose of the Secondary Air Injection System?

It’s no secret that gas-powered engines aren’t 100% efficient. As a result, there is a portion of unburned fuel (hydrocarbons) that doesn’t 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. Secondly, it takes a couple of minutes for the engine and catalytic converters to get to operating temperatures.

These two factors combine to increase the amount of unburned fuel after a cold start. Thus, the purpose of the secondary air injection system is reducing hydrocarbon and carbon monoxide emissions that result from cold start conditions.

What Components Comprise the Secondary Air Injection System?

There are several components that comprise the secondary air injection system. They include air injection drivers, air pumps, air switching valves, and pressure sensors. The location of these components 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 reducing emissions after a cold start by burning fuel wasn’t combusted by the engine. How is this accomplished? As we know, 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 SAIS.

On cold start ignitions, the air injection drivers will command the air switching valves to open and the air pumps to pump air through the air switching valves. The air is then injected into the exhaust manifold upstream 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, helping the catalytic converters heat up to operating temperature quicker. It typically takes 30 to 90 seconds to get to operating temperature, after which air from the SAIS is no longer needed and the SAIS stops operating. The air pumps turn off and the air switching valves close. If the air switching valve does not close, exhaust can leak into the SAIS and cause additional issues.

Can You Drive Without the Secondary Air Injection System?

How does it affect the vehicle if the secondary air injection system is not working properly? Mechanically speaking, the secondary air injection system only affects post-combustion operations, and so a nonfunctioning SAIS doesn’t 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 at least seven hours. Even then, it will only run for 30 to 90 seconds to allow the operating temperature to be reached.

It would be more accurate to state that the SAIS reduces emissions at most once every seven hours for 30 to 90 seconds. Outside of that specific time period, the SAIS is inactive. Consequently, if the secondary air injection system is not working properly, 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 mode,” which is a reduced performance mode where power and acceleration are limited

Additional Secondary Air Injection Resources

To learn more about the Hewitt Tech bypass kits or fault codes for the secondary air injection system, click on the following “Must Read” Hewitt Tech blogs:

Contact Us

If you have questions about compatibility or the solution that is best for you, do not hesitate to contact us. You can reach Customer Service and Technical Support at (844) 307-7671, Monday through Friday from 8:30 a.m. to 5:00 p.m. ET. Feel free to send us a message through our Contact Us page. Hewitt is here to help.


Hewitt Tech’s SAIS bypass kits and products are sold for off-road, competition, or other emissions-exempt vehicles only. It is illegal to remove, dismantle, or otherwise cause to be inoperative any pollution control device required by federal, state, or local emissions law. No other applications are intended or implied. By installing or using this SAIS bypass kit, the vehicle owner and installer acknowledges and assumes ALL risks associated with its use.


  1. michael on April 25, 2023 at 4:27 pm

    The support and help from Derek and the team at Hewitt Technologies are top-class and very professional. The issue with my Toyota Landcruiser Prado all the way down under here in New Zealand was fixed and my Prado running like it should with the recommended Gen-2 bypass kit, even after having further issues Derek was super helpful and got it sorted, I can strongly recommend Hewitt Tecnologies to all for Fast and Professional help and support anywhere in the world.
    Cheers Derek.
    Michael from New Zealand

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

    After using your bypass kit more than a year now, I can say that I am extremely happy that I got a bypass kit from you guys. It is working like a champ. Thanks again for your service and products.

    • Derek Bley on April 10, 2023 at 9:23 am

      Jose your review is the best “Thank You” we could receive! There is nothing we like better than happy customers and vehicles running like there suppose to, if there is anything else we can assist with please do not hesitate to contact us.

Leave a Comment

You must be logged in to post a comment.