What is the code P0412 on a Toyota OBD II?

The Toyota OBD2 code P0412 is a diagnostic trouble code that indicates an issue with the secondary air injection system. This code is triggered when the engine control module (ECM) detects a problem with the operation or function of the secondary air injection system.

What is the Secondary Air Injection System?

The secondary air injection system is a component of the emissions control system in many modern vehicles, including Toyota models. Its primary function is to introduce additional air into the exhaust stream to help oxidize unburnt fuel and reduce harmful emissions, particularly during cold starts and low-load conditions.

The secondary air injection system typically consists of an air pump, one-way valves, and various sensors and wiring. When the system is functioning correctly, the air pump forces air into the exhaust manifold, where it mixes with the exhaust gases and undergoes further combustion, reducing the amount of unburnt hydrocarbons and carbon monoxide released into the atmosphere.

What Causes the P0412 Code?

The P0412 code is triggered when the ECM detects an issue with the secondary air injection system. Some common causes of this code include:

• Faulty air pump: The air pump may be malfunctioning, not providing the necessary airflow to the exhaust system.


• Blocked or stuck one-way valves: The one-way valves that allow air to flow into the exhaust but prevent backflow can become stuck or blocked, preventing proper system operation.


• Wiring or electrical issues: Problems with the wiring, connectors, or electrical components of the secondary air injection system can cause the P0412 code to be set.


• Sensor issues: Malfunctioning sensors, such as the air flow sensor or pressure sensor, can also trigger the P0412 code.

If you receive a P0412 code, it's important to have the vehicle diagnosed by a qualified technician to identify and address the underlying issue.

Diagnosing and Resolving the P0412 Code

To diagnose and resolve the P0412 code, a technician will typically follow these steps:

1. Perform a visual inspection: Check for any obvious signs of damage or wear on the secondary air injection system components, such as the air pump, one-way valves, and wiring.


2. Check for diagnostic trouble codes: Scan the vehicle's onboard diagnostics system to see if any other codes are present, as they may provide additional clues about the root cause of the P0412 code.


3. Test the air pump and one-way valves: The technician will likely test the air pump's operation and the one-way valves to ensure they are functioning correctly.


4. Inspect the wiring and electrical components: The technician will check the wiring, connectors, and electrical components of the secondary air injection system for any issues.


5. Replace faulty components: If the issue is identified, the technician will replace the faulty component(s) to resolve the P0412 code.

Once the necessary repairs have been made, the technician will clear the diagnostic trouble codes and test the vehicle to ensure the P0412 code does not reappear.

What causes secondary air injection system malfunction?

Secondary air injection system malfunctions are caused by issues with the air pump, faulty valves or relays, air leaks from damaged hoses, and electrical problems like a blown fuse or bad wiring. Moisture infiltration is a common culprit, leading to rust and seizing of components like the combination valve. Electrical issues, worn-out parts, and blockages from carbon buildup can also cause the system to fail. 
Component failures

• Air pump: Often fails due to moisture getting inside and causing it to seize, or from general wear and tear. 
• Valves: The combination valve (or non-return valve) can fail due to rust, while other valves like check valves can become stuck or clogged. 
• Relay: A faulty or "jammed" air pump relay can cause the pump to run for too long or not at all. 
• Relay contacts: Corroded or damaged relay contacts can prevent proper operation. 

Air leaks and blockages

• Damaged hoses: Rubber hoses can crack or become brittle from heat, causing air leaks. Inspecting them can often cause them to split further. 
• Vacuum lines: Damaged or leaking vacuum lines can disrupt the control of the system. 
• Clogged passages: Carbon buildup can block the air injection ports in the cylinder heads or other passages, impeding airflow. 

Electrical problems

• Faulty wiring: Damaged wiring, connectors, or a bad ground can prevent the system from working correctly. 
• Blown fuse: A simple blown fuse is a common and easy-to-fix electrical problem. 
• Faulty sensors: A malfunctioning oxygen sensor can send incorrect data to the engine control module (ECM), leading to system errors. 
• Failed ECM: Less commonly, the engine control module itself can fail. 

How to fix P0492 code?

Multiple repairs may be necessary to repair the P0492 code. The most common repairs are replacement of the pump and the one-way check valve. However, vacuum line leaks will also need to be repaired, and the cylinder head may need to be thoroughly cleaned of carbon build up before the code can be cleared permanently.

What is the P0412 circuit of secondary air injection system switching valve A?

The P0412 code indicates a problem with the "A" circuit of the secondary air injection (SAI) system, likely caused by an open or short in the wiring, a faulty switching valve, or a failing air pump. The SAI system helps reduce emissions by injecting fresh air into the exhaust on cold starts. To diagnose and fix this, check the wiring harness for damage, test the air switching valve and air pump for proper function, and replace any blown fuses. 
Causes of the P0412 code

• Wiring and harness issues: Open or short circuits, or corroded connectors, in the SAI system's wiring are a common cause. 
• Faulty air switching valve: The valve itself may be damaged or not functioning correctly. 
• Failing air pump: The secondary air pump could be malfunctioning or clogged. 
• Damaged hoses: Leaks or damage to the vacuum hoses can trigger the code. 
• Blown fuses: A short circuit can blow a fuse in the system. 
• Powertrain Control Module (PCM) failure: Though less common, the vehicle's computer could be the source of the problem. 

This video explains how to diagnose and fix the P0412 code on Toyota vehicles: 37snondaYouTube · Feb 26, 2021
How to fix the P0412 code

1. Check and clear blown fuses: Inspect and replace any blown fuses in the SAI system. A code reader can be used to clear the code after replacement. 
2. Inspect the air pump and switching valve: 
   • Listen for unusual noises from the air pump. 
   • Inspect the pump and switching valve for any physical damage or corrosion. 
   • Disconnect the air switching valve connector and test the resistance of the wiring harness side to check for shorts or opens, as shown in this Toyota manual document. 
3. Inspect the wiring harness: Check the entire wiring harness for any signs of damage, such as breaks, fraying, or corrosion. Repair any damaged sections. 
4. Test the components: If wiring is not the issue, test the air pump and switching valve for proper operation using a diagnostic tool or multimeter. 
5. Replace faulty components: Based on your findings, replace any faulty components, such as the air pump, switching valve, or air check valve. 
6. Clear the code: After repairs are complete, use a code reader to clear the P0412 code from the system. 

How do I fix code P0412?

How to Diagnose and Fix the P0412 Code

1. Consult vehicle-specific repair manual for diagnostic procedures.
2. Inspect SAI system components for damage.
3. Check for loose or corroded wiring connections.
4. Ensure proper vacuum hose connections.
5. Perform a visual inspection of related sensors and valves.