P2259 Secondary Air Injection System Control "B" Circuit Low

P2259 is a generic OBD-II diagnostic trouble code that indicates the Engine Control Module (ECM) has detected a low voltage condition in the Secondary Air Injection (SAI) System Control "B" Circuit.

In simple terms, the ECM is receiving a voltage signal from the Secondary Air Injection System Control "B" circuit that is lower than the manufacturer's specified operating range. Because of this abnormal signal, the ECM cannot properly control the secondary air injection system, which helps reduce exhaust emissions during cold engine starts.

The Secondary Air Injection (SAI) system injects fresh air into the exhaust manifold or exhaust ports immediately after a cold start. The additional oxygen helps burn unburned fuel in the exhaust, allowing the catalytic converter to warm up faster and reducing harmful emissions.

When the ECM detects a low voltage condition in the control circuit, it stores diagnostic trouble code P2259 and usually illuminates the Check Engine Light.

What Does P2259 Mean?

The Secondary Air Injection System is part of the vehicle's emissions control system and is commonly found on gasoline engines.

Its primary purpose is to reduce hydrocarbon (HC) and carbon monoxide (CO) emissions during cold starts before the catalytic converter reaches operating temperature.

The system typically includes:

• Secondary air injection pump

• Air injection control valve

• Air switching valve

• Vacuum control solenoid (on some vehicles)

• Relay

• Wiring harness

• Engine Control Module (ECM)

The ECM activates the air pump and opens the control valves when cold-start conditions require additional airflow into the exhaust system.

The code description contains several important meanings:

Secondary Air Injection System

An emissions control system that injects fresh air into the exhaust system during cold engine operation.

Control "B"

The letter "B" identifies a specific control circuit, valve, relay, or actuator used by the manufacturer. Its exact location varies depending on the vehicle.

Circuit

The circuit includes:

• Power supply

• Ground circuit

• Signal wire

• Electrical connectors

• ECM communication

Low

The ECM detects a voltage signal that is below the expected operating range.

Possible causes include:

• Short to ground

• Open power supply

• Faulty control valve

• Damaged wiring

• Poor electrical connections

Symptoms of P2259

Many vehicles continue to drive normally with P2259, although emissions performance may be affected.

Common symptoms include:

Check Engine Light

The Check Engine Light usually illuminates after the ECM detects the fault.

Increased exhaust emissions

The Secondary Air Injection System may fail to operate during cold starts.

Failed emissions inspection

The vehicle may fail an emissions or inspection test.

Rough cold idle

Some engines may idle roughly for a short time after starting.

Poor cold-start performance

Cold engine operation may become less stable until the catalytic converter reaches operating temperature.

Reduced fuel efficiency

Although uncommon, slightly increased fuel consumption may occur on some vehicles.

Common Causes of P2259

Several problems can cause a Secondary Air Injection System Control "B" Circuit Low condition.

Faulty secondary air injection control valve

A defective control valve is one of the most common causes.

Possible failures include:

• Internal electrical failure

• Open circuit

• Sticking valve

Short to ground

The control circuit may be contacting the vehicle chassis or another ground source.

Possible causes include:

• Damaged insulation

• Chafed wiring

• Melted wires

Open power supply

If the control valve loses battery voltage, the ECM may detect a low circuit signal.

Possible causes include:

• Broken power wire

• Blown fuse

• Loose electrical connection

Damaged wiring harness

The wiring may become damaged due to:

• Heat exposure

• Engine vibration

• Abrasion

• Rodent damage

Corroded or loose connectors

Poor electrical connections can reduce circuit voltage.

Possible issues include:

• Corrosion

• Bent connector pins

• Loose terminals

• Moisture intrusion

Faulty relay

Many Secondary Air Injection systems use a dedicated relay.

A failed relay may interrupt power to the control circuit.

Faulty secondary air injection pump

Although less common, an internally failed pump may affect the control circuit.

ECM problems

In rare cases, the ECM may incorrectly detect a low voltage condition.

Possible causes include:

• Software issues

• Internal ECM failure

Vehicles Commonly Affected by P2259

P2259 may occur on many gasoline-powered vehicles equipped with Secondary Air Injection systems.

Common examples include:

• BMW gasoline models

• Mercedes-Benz gasoline vehicles

• Audi gasoline models

• Volkswagen gasoline vehicles

• Porsche models

• Toyota vehicles equipped with SAI systems

• Lexus vehicles

• Ford gasoline vehicles

• General Motors gasoline engines

• Various European, Asian, and American vehicles

The exact location of the "B" control circuit varies by manufacturer.

How Is P2259 Diagnosed?

Proper diagnosis requires inspecting both the electrical circuit and the Secondary Air Injection system.

Check for additional trouble codes

A professional scan tool should check for related trouble codes such as:

• Secondary Air Injection pump codes

• Air switching valve codes

• Relay control codes

• Oxygen sensor codes

These codes may help identify the root cause.

Inspect the control valve

The control valve should be inspected for:

• Physical damage

• Internal sticking

• Proper electrical resistance

• Secure installation

Inspect wiring and connectors

Technicians should inspect the circuit for:

• Broken wires

• Short-to-ground conditions

• Corrosion

• Loose connectors

• Heat damage

Test the relay

If equipped, the relay should be tested for:

• Proper operation

• Correct voltage output

• Contact condition

Monitor live data

Using a professional scan tool, technicians should monitor:

• Commanded Secondary Air Injection operation

• Actual circuit voltage

• Pump activation

• Valve response

Test the air injection pump

The pump should be tested for:

• Proper operation

• Electrical current draw

• Airflow output

How to Fix P2259

The proper repair depends on the underlying cause of the low voltage condition.

Common repairs include:

Replace the secondary air injection control valve

A faulty control valve should be replaced if testing confirms failure.

Repair damaged wiring

Electrical repairs may include:

• Replacing damaged wires

• Repairing short-to-ground faults

• Restoring damaged insulation

Repair connector problems

Repairs may include:

• Cleaning corroded terminals

• Replacing damaged connectors

• Tightening loose electrical connections

Replace the relay

A defective relay should be replaced if it fails testing.

Replace the secondary air injection pump

A failed pump should be replaced if necessary.

Repair the power supply circuit

Blown fuses, damaged power wires, or poor electrical connections should be repaired.

Repair ECM-related faults

Rarely, ECM software updates or replacement may be required.

After repairs:

1. Clear the diagnostic trouble code.

2. Start the engine from cold.

3. Verify proper Secondary Air Injection system operation.

4. Perform a road test.

5. Confirm that the code does not return.

Can You Drive With P2259?

Driving with P2259 is generally possible because the Secondary Air Injection System mainly affects emissions rather than basic engine operation.

However, possible consequences include:

• Increased exhaust emissions

• Failed emissions inspection

• Check Engine Light remaining illuminated

• Reduced cold-start emissions performance

Although the vehicle may continue to operate normally, repairs should not be delayed.

Is P2259 a Serious Code?

P2259 is generally considered a moderate diagnostic trouble code.

It typically does not cause immediate engine damage, but it can:

• Increase harmful exhaust emissions

• Prevent proper Secondary Air Injection operation

• Cause emissions inspection failures

• Keep the Check Engine Light illuminated, potentially masking additional engine problems

Timely diagnosis helps restore emissions system performance and maintain regulatory compliance.

How to Prevent P2259

Routine maintenance can reduce the likelihood of Secondary Air Injection System faults.

Recommended practices include:

• Inspect the air injection system wiring regularly.

• Keep electrical connectors clean and protected from moisture.

• Repair damaged wiring immediately.

• Replace worn relays when necessary.

• Inspect vacuum hoses on vacuum-operated systems.

• Address Check Engine Light warnings promptly.

• Follow the manufacturer's recommended maintenance schedule.

Final Thoughts

P2259 Secondary Air Injection System Control "B" Circuit Low indicates that the ECM has detected a voltage signal in the Secondary Air Injection System Control "B" circuit that is below the expected operating range.

The most common causes include a faulty control valve, short-to-ground wiring faults, an open power supply, damaged electrical connectors, a defective relay, wiring damage, or, in rare cases, an ECM malfunction.

A complete inspection of the Secondary Air Injection system, including the control valve, relay, wiring, connectors, and pump, is essential to restore proper emissions control, ensure reliable cold-start performance, and maintain compliance with emissions regulations.