P1490

What Does Code P1490 Mean?

The diagnostic trouble code P1490 signifies a “Secondary Air Relief Solenoid Circuit Malfunction”. This code indicates that the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), has detected an electrical fault within the circuit controlling the secondary air injection (SAI) relief solenoid. The Secondary Air Injection system is an emissions control component designed to reduce exhaust emissions during cold engine starts. During this phase, the engine runs rich, producing higher levels of unburnt hydrocarbons and carbon monoxide. The SAI system injects fresh, oxygen-rich air directly into the exhaust manifold or upstream of the catalytic converter, facilitating an exothermic reaction that burns these pollutants more completely before the catalytic converter reaches its optimal operating temperature. The relief solenoid, depending on the specific vehicle’s SAI design, typically controls a valve that either diverts air pressure from the SAI pump, or releases pressure from the system, or directs air flow to specific locations (e.g., manifold vs. catalytic converter input) to optimize emissions reduction. The ECM monitors the electrical characteristics of this solenoid circuit, such as voltage, current draw, and resistance. When the ECM detects an unexpected electrical value – for instance, an open circuit (infinite resistance), a short to ground (zero resistance), a short to voltage, or an out-of-range current draw – it interprets this as a malfunction in the solenoid’s control circuit and sets the P1490 code, illuminating the Malfunction Indicator Lamp (MIL).

Common Symptoms

• Illuminated Malfunction Indicator Lamp (MIL): The “Check Engine” light will be on.
• Failed Emissions Inspection: Due to the SAI system not operating correctly, the vehicle may fail a state-mandated emissions test.
• No Noticeable Driveability Issues: In most cases, the P1490 code will not result in any perceptible changes in engine performance, fuel economy, or driveability, as the SAI system primarily operates during cold start cycles and has minimal impact once the engine is at operating temperature.
• Increased Cold Start Emissions: Though not directly noticeable by the driver, the vehicle will emit higher levels of pollutants during initial engine operation.

What Causes the Code P1490?

• Faulty Secondary Air Relief Solenoid: The most common cause, where the solenoid’s internal coil may be open, shorted, or mechanically stuck.
• Wiring Harness Issues: An open circuit, short to ground, or short to voltage within the wiring harness connecting the solenoid to the ECM. Damage can occur from chafing, heat, or rodent intrusion.
• Poor Electrical Connections: Corroded, loose, or damaged connector pins at the secondary air relief solenoid, or at the ECM connector, leading to high resistance or intermittent contact.
• Blown Fuse: The solenoid circuit may be protected by a fuse which could have blown, interrupting power supply.
• Faulty Engine Control Module (ECM/PCM): Although less common, an internal failure within the ECM’s driver circuit for the secondary air relief solenoid can cause this code.

How to Diagnose and Troubleshoot

Diagnosing P1490 requires a methodical approach using a digital multimeter (DMM) and an OBD-II scanner with enhanced capabilities.

1. Visual Inspection: Begin with a thorough visual inspection of the secondary air relief solenoid, its electrical connector, and the associated wiring harness. Look for obvious signs of damage, such as frayed wires, melted insulation, corrosion on connector pins, or loose connections. Inspect any vacuum lines connected to the solenoid or related valves, if applicable to the system design, for cracks or disconnections. Locate and inspect the fuse for the SAI system; if blown, replace it, but be aware that a shorted solenoid or wiring could cause it to blow again.
2. OBD-II Scanner Data: Connect an OBD-II scanner and verify the P1490 code. Note any other co-existing codes, as they may provide additional diagnostic direction. If the scanner supports bi-directional control, attempt to command the secondary air relief solenoid ON and OFF while monitoring its circuit status (if available) or listening for an audible click from the solenoid itself.
3. Secondary Air Relief Solenoid Resistance Test: Disconnect the electrical connector from the secondary air relief solenoid. Using a DMM set to ohms, measure the resistance across the two terminals of the solenoid. Compare this reading to the manufacturer’s specifications (typically found in a service manual). A common range for solenoids is between 10 to 50 ohms. An “OL” (open loop/infinite resistance) reading indicates an open coil within the solenoid, while a very low reading (close to 0 ohms) indicates an internal short. In either case, the solenoid is faulty.
4. Power and Ground Circuit Test:
   • Power Supply: With the ignition ON (and engine OFF), use the DMM set to DC volts. Identify the power supply wire to the solenoid. This wire should show battery voltage (typically 12V). If no voltage is present, trace the wire back to its power source (e.g., fuse box) to identify an open circuit or blown fuse.
   • Ground Control (if applicable): If the ECM controls the solenoid by providing a ground signal, verify the power supply side first. Then, with the solenoid connected and commanded ON (using the scanner’s bi-directional control), check for a ground signal at the control wire from the ECM.
   • Continuity to Ground/Voltage: Disconnect the ECM connector and the solenoid connector. Use the DMM set to ohms to check for continuity between the solenoid’s power and ground wires and chassis ground or battery positive. Any continuity indicates a short circuit that needs to be located and repaired.
5. ECM Driver Test (Advanced): If all solenoid and wiring checks prove satisfactory, the issue may lie within the ECM’s driver circuit. This requires specialized tools or knowledge to confirm, often involving scope patterns of the ECM’s control signal. ECM failure is rare and should only be considered after all other potential causes have been thoroughly ruled out.

Recommended Repairs and Solutions

Once the root cause of the P1490 code has been accurately diagnosed, the appropriate repair can be implemented:

• Replace Secondary Air Relief Solenoid: If DMM tests confirm the solenoid’s internal coil is faulty (open or shorted), or if it is mechanically stuck, replacement is necessary. Ensure to use a direct OEM replacement or a high-quality aftermarket equivalent.
• Repair or Replace Wiring Harness: If damaged or corroded wiring is identified, repair the affected section using appropriate automotive-grade wiring, connectors, and heat-shrink tubing. If the damage is extensive, replacing the entire sub-harness may be more practical. Always ensure secure and weather-tight connections.
• Address Connector Issues: Clean any corroded electrical connectors using electrical contact cleaner and a small brush. If pins are bent or damaged, attempt to carefully straighten them or replace the connector housing and pins if severely compromised. Apply a small amount of dielectric grease to the connector after cleaning to prevent future corrosion.
• Replace Blown Fuse: If a blown fuse was the cause, replace it with a fuse of the correct amperage. However, it is crucial to investigate why the fuse blew, as it often indicates a short circuit elsewhere in the system (e.g., a shorted solenoid or wiring). Simply replacing the fuse without addressing the underlying cause will likely result in a repeat failure.
• ECM/PCM Replacement or Repair: If all other components and wiring have been thoroughly tested and verified as functional, and a fault within the ECM’s driver circuit is suspected, then ECM replacement or repair may be required. This is a complex and costly repair, often requiring reprogramming and re-learning procedures specific to the vehicle, and should be performed by a qualified technician.

Mechanic’s Tips: Always disconnect the vehicle’s battery before performing any electrical repairs to prevent accidental shorts or damage to electrical components. After completing repairs, clear the P1490 code using an OBD-II scanner and perform a drive cycle, including a cold start, to confirm the repair has resolved the issue and the SAI system operates correctly. Verify that all system monitors reset to “ready” status for emissions compliance.