Faulty secondary air switching (combination or shut-off) valve solenoid or actuator; Damaged, chafed or corroded wiring between the valve and the control unit (open circuit or short to earth or supply); Corroded, loose or water-contaminated connector at the valve or control unit; Blown fuse or faulty relay supplying the valve circuit (depends on model and system layout); Valve seized or clogged with moisture or carbon, drawing abnormal current; Fault within the engine control unit's output driver.

P0412

AIR System Switching Valve A Circuit.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
4: AreaAuxiliary emission controls
12: FaultThe specific fault in that area

What it means

P0412 is the generic code for a fault in the electrical circuit of switching valve A in the secondary air injection (AIR) system. The engine control unit monitors the valve's control circuit and sets the code when the voltage or current it sees does not match the commanded state, which points to an open circuit, a short or a failed valve. The code relates to the circuit, not necessarily to airflow, though a faulty valve can also affect the secondary air system's operation.

Symptoms

Engine warning light on
Possible failed emissions test
Secondary air pump running at the wrong time or not at all (may be heard or not heard at cold start)
Slightly rough or unusual idle on cold start in some cases
Other secondary air system codes stored alongside it
Often no noticeable change in driveability

Possible causes

Faulty secondary air switching (combination or shut-off) valve solenoid or actuatorlikely
Damaged, chafed or corroded wiring between the valve and the control unit (open circuit or short to earth or supply)likely
Corroded, loose or water-contaminated connector at the valve or control unitlikely
Blown fuse or faulty relay supplying the valve circuit (depends on model and system layout)possible
Valve seized or clogged with moisture or carbon, drawing abnormal currentpossible
Fault within the engine control unit's output driverrare

How to diagnose

Read all stored codes and freeze frame data, noting any other secondary air or pump codes, and check for relevant manufacturer service bulletins for the model
Visually inspect the valve, its vacuum or electrical connector and the wiring for damage, heat, rubbing or water ingress
Check the relevant fuse and relay against the wiring diagram for that vehicle
Measure the valve solenoid resistance and compare it with the manufacturer's specification for that engine
With the connector unplugged, check for supply voltage and a good earth or control signal, and test the wiring for continuity and shorts
Command the valve using a bidirectional scan function if supported and check it responds, listening and feeling for operation
Inspect the valve and associated pipework for moisture, carbon blockage or a stuck mechanism
If all wiring and the valve test fine, check the control unit output signal with a scope and consider a control unit fault

How to fix

Replace the faulty switching valve or its solenoid assembly
Repair or replace damaged wiring and ensure it is properly routed and secured
Clean, repair or replace corroded connectors and apply appropriate protection against moisture
Replace the blown fuse after finding why it failed, or replace a faulty relay
Clear any blockage or replace a seized valve and check the pump and pipework for moisture damage
Repair or replace the control unit only after all other causes are ruled out

Can I keep driving?

Low severity

The car can normally be driven while you book it in, as the fault rarely affects safety or driveability. Get it fixed reasonably soon, because the secondary air system is part of the emissions control and the fault will likely cause an MOT emissions failure.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.