P2491 – Secondary Air Injection System Control Circuit Low

System: Powertrain | Standard: ISO/SAE Controlled | Fault type: Circuit Low

Definition source: SAE J2012/J2012DA (industry standard)

P2491 is a powertrain diagnostic trouble code that indicates the control circuit for the secondary air injection (SAI) system is being detected as “low” by the vehicle’s control module. In practical terms, the module is seeing an electrical condition consistent with low input on the circuit it uses to command or monitor SAI operation. The exact circuit design, components involved, and enabling conditions for setting the code vary by vehicle, so use the correct service information for wiring diagrams, connector views, and test specifications before making repairs.

What Does P2491 Mean?

P2491 – Secondary Air Injection System Control Circuit Low means the engine control module has identified a low electrical condition in the secondary air injection system control circuit. This is an electrical fault type (circuit low), not a guaranteed mechanical failure. SAE J2012 defines how DTCs are structured and categorized; the “circuit low” wording indicates the module’s observed signal/command feedback is lower than expected for the conditions under which the test runs. The specific actuator, relay/driver, or monitored line that constitutes the “control circuit” can vary by vehicle.

Quick Reference

• System: Powertrain
• Official meaning: Secondary Air Injection System Control Circuit Low
• Standard: ISO/SAE controlled
• Fault type: Circuit Low
• Severity: The MIL may illuminate; drivability is often limited, but emissions impact is likely and some platforms may alter cold-start strategy.

Symptoms

• MIL/Check engine light: Lamp illuminated with P2491 stored (pending or confirmed).
• Cold-start behavior: Roughness or unstable idle may be noticed during initial start-up when SAI is typically commanded (varies by vehicle).
• Emissions readiness: SAI-related monitors may not complete, causing an inspection/readiness failure.
• Secondary air system inactivity: SAI pump/valve may not run when commanded, depending on system design and fail-safe strategy.
• Additional DTCs: Related circuit, relay/driver, or secondary air codes may appear alongside P2491.
• Intermittent warning: The code may set intermittently due to vibration-sensitive wiring/connector faults.

Common Causes

• Short-to-ground in the control circuit: Damaged insulation or pinched harness pulling the command/feedback line low.
• Open power feed to the secondary air injection actuator: Blown fuse, open relay feed, or broken conductor preventing the circuit from reaching the expected level.
• High resistance in wiring: Corrosion, partially broken strands, or poor splices creating excessive voltage drop and a low signal condition.
• Poor ground path: Loose ground fastener, corrosion at ground point, or ground conductor damage causing improper circuit operation and low reading.
• Connector faults: Bent pins, backed-out terminals, water intrusion, or poor terminal tension at the pump/valve/relay/ECM connectors (varies by vehicle).
• Relay control-side fault: Relay coil shorted, incorrect relay, or relay socket terminal damage loading the control circuit low.
• Secondary air injection actuator issue: Pump, solenoid, or control valve electrical fault drawing the circuit low (component type varies by vehicle).
• Control module driver/logic issue: Rare, but a failing driver transistor or internal circuit fault can hold the control circuit low after wiring and loads are verified.

Diagnosis Steps

Tools that help include a scan tool with live-data logging and bi-directional controls (if supported), a digital multimeter, and basic back-probing leads. A wiring diagram and connector views for the secondary air injection control circuit are essential because routing and control strategy vary by vehicle. Use supplies for connector inspection/cleaning and plan time for a harness wiggle test and voltage-drop checks under load.

1. Confirm the code and capture freeze-frame: Scan for P2491 and record freeze-frame data and any related DTCs. Address battery/voltage or communication codes first if present, because they can bias circuit diagnostics.
2. Verify the complaint and attempt an active test: If bi-directional control is available, command the secondary air injection function on/off and observe any related PIDs (command state, feedback where available). If active control is not supported, proceed with circuit checks using service information.
3. Initial visual inspection: Key off. Inspect the harness from the control module to the secondary air injection actuator/relay (varies by vehicle). Look for chafing near brackets, heat damage, crushed sections, prior repairs, and signs of water intrusion at connectors.
4. Check fuses and relay power feeds: Verify the circuit’s power supply path(s) per wiring diagram. If a fuse is blown, do not replace it repeatedly—first inspect for a short-to-ground or a seized electrical load that could be pulling the circuit low.
5. Connector and terminal integrity checks: Disconnect relevant connectors and inspect for bent pins, corrosion, pushed-out terminals, and poor pin fit. Lightly tug-test suspect wires at the back of the connector. Repair terminal issues before deeper electrical testing.
6. Test for short-to-ground on the control circuit: With connectors disconnected as directed by service info, check the control circuit for continuity to ground. An unexpected ground path indicates a harness short, water intrusion, or an internally shorted component on that circuit.
7. Check for open power/feed or open control circuit: Verify continuity end-to-end on the control wire(s) and power feed(s) per diagram. If continuity is intermittent, flex the harness while monitoring to pinpoint a break or poor splice that can cause a low circuit condition under vibration.
8. Voltage-drop test under load: Reconnect the circuit and command the system on (or run the condition when it operates, per service info). Perform voltage-drop checks across power and ground paths to find high resistance that can pull circuit levels low. Focus on fuse/relay sockets, splices, ground points, and connector pins.
9. Load isolation: If the circuit reads low only when a device is connected, isolate loads one at a time (relay coil, pump/solenoid/valve, etc., as applicable). A component with internal electrical fault can load the control circuit low; confirm by comparing behavior connected vs disconnected per service procedures.
10. Wiggle test with live logging: While logging relevant PIDs and/or monitoring the control circuit, wiggle the harness and tap lightly near connectors and relay/fuse boxes. If the signal drops low or the DTC resets, pinpoint the location and repair the mechanical/electrical fault.
11. Evaluate control module command/driver last: After verifying power, ground, wiring integrity, and component loads, check whether the control module is commanding the circuit appropriately. If the driver appears to hold the circuit low with all external factors eliminated, follow service info for module pin testing and replacement/programming requirements.
12. Clear codes and verify the repair: Clear DTCs, run the enabling conditions/drive cycle per service information, and confirm P2491 does not return. Re-scan for pending codes to ensure the low-circuit condition is fully resolved.

Professional tip: A “circuit low” fault is often caused by voltage drop or an unintended ground path that only shows up when the circuit is loaded. Prioritize testing with the circuit operating (commanded on) and use voltage-drop measurements rather than only checking continuity; a wire can pass a continuity test yet still drop enough voltage under load to trigger P2491.

Possible Fixes & Repair Costs

Repair costs can vary widely because the root cause may be as simple as a wiring issue or as involved as replacing an actuator or module. Parts pricing, diagnostic time, and labor access differ by vehicle, so confirm the failure with testing before replacing components.

• Repair wiring faults: Restore damaged insulation, broken conductors, or chafed harness sections in the secondary air injection control circuit.
• Clean and secure connectors: Remove corrosion, correct poor terminal tension, and ensure full connector seating/lock engagement at the control circuit endpoints.
• Restore power/ground integrity: Replace a blown fuse (after finding the cause), repair an open feed, or correct a high-resistance ground causing a low signal condition.
• Replace the failed control component: If testing proves the commanded output cannot be driven high due to an internal fault, replace the secondary air injection control device used on that platform (varies by vehicle).
• Replace the secondary air injection pump/valve/solenoid only if verified: If the load is shorted internally and is pulling the control circuit low, replace the confirmed faulty component.
• Repair relay and relay socket issues: Replace a sticking/failed relay and repair overheated or loose relay terminals if the circuit design uses a relay (varies by vehicle).
• Address water intrusion: If testing finds moisture in connectors or harness segments causing a short-to-ground path, dry/repair and restore sealing.

Can I Still Drive With P2491?

In many cases the vehicle may remain drivable, but P2491 indicates a control circuit low condition that can prevent the secondary air injection system from operating as intended and may illuminate the MIL. If you experience stalling, no-start, reduced power, or any safety-related warnings (such as brake or steering alerts), do not continue driving; have the vehicle inspected and the electrical fault confirmed with service information for your platform.

What Happens If You Ignore P2491?

Ignoring P2491 can lead to repeated MIL illumination, failed emissions readiness/inspection, and ongoing electrical stress if the low condition is caused by a short-to-ground or high resistance. A persistent circuit fault can also contribute to additional diagnostic trouble codes and make future troubleshooting more time-consuming by masking other issues.

Related Secondary Air Codes

Compare nearby secondary air trouble codes with similar definitions, fault patterns, and diagnostic paths.

• P2449 – Secondary Air Injection System Switching Valve Control Circuit Low
• P2060 – Reductant Injection Air Pump Control Circuit Low
• P2450 – Secondary Air Injection System Switching Valve Control Circuit High
• P2492 – Secondary Air Injection System Control Circuit Range/Performance
• P2490 – Secondary Air Injection System Control Circuit High
• P0541 – Intake Air Heater “A” Circuit Low

Key Takeaways

• P2491 is electrical: It indicates a secondary air injection system control circuit low condition, not a confirmed mechanical failure by itself.
• Most common roots: Shorts-to-ground, open power/feed, poor grounds, and connector/terminal problems are typical starting points.
• Test before replacing: Use circuit checks, voltage-drop testing, and command/feedback verification to confirm the exact fault location.
• Emissions impact: The issue can prevent proper secondary air injection operation and may affect inspection readiness.
• Design varies: The exact actuator/relay/module arrangement differs by vehicle, so follow service information for pinouts and test procedures.

Vehicles Commonly Affected by P2491

• Vehicles equipped with secondary air injection: Platforms that use an air pump and switching/combination valves for cold-start emissions control.
• Systems with relay-driven air pumps: Architectures that switch pump power through a relay commanded by a controller output.
• Systems using solenoids or valves: Designs where a control circuit drives a solenoid/valve that can short internally and pull the circuit low.
• High-heat engine bays: Layouts where harness routing near exhaust components increases the chance of insulation damage and shorts-to-ground.
• Vehicles exposed to moisture: Environments where connectors, relay boxes, or harness splices can corrode and create low-signal conditions.
• Vehicles with prior wiring repairs: Platforms with aftermarket splices, prior collision repair, or harness work that can introduce high resistance or mispinned connectors.
• High-mileage vehicles: Aging terminals, weakened pin tension, and brittle wiring can increase the likelihood of intermittent-to-hard low faults.
• Vehicles with underhood electrical distribution blocks: Fuse/relay centers that can develop poor contact or heat damage affecting control circuit integrity.

FAQ

Is P2491 telling me the secondary air injection pump is bad?

No. P2491 indicates the secondary air injection system control circuit is being detected low. That can be caused by wiring, connectors, power/ground problems, a shorted load, or a control device fault; the pump or valve should only be replaced if testing proves it is pulling the circuit low or otherwise failing electrically.

What does “control circuit low” usually mean in electrical terms?

It generally points to an electrical low input/low voltage condition on the monitored control circuit, commonly due to a short-to-ground, an open power/feed, excessive resistance causing voltage drop, or a driver/output that cannot raise the circuit as commanded. Exact detection logic varies by vehicle.

Will clearing the code fix the issue?

Clearing the code only resets stored information; it does not repair the underlying circuit condition. If the low condition is still present, the code will typically return after the next self-test or when the system is commanded on.

Can a blown fuse cause P2491?

Yes, depending on how the circuit is designed. An open fuse or lost power feed can prevent the circuit from reaching the expected level and be interpreted as a low condition. If a fuse is blown, diagnose the reason it opened before replacing it.

What should I check first to diagnose P2491 efficiently?

Start with a visual inspection of the secondary air injection control circuit wiring and connectors, then verify the power and ground paths under load using voltage-drop testing. If available, command the system on with a scan tool and observe whether the control circuit transitions appropriately; follow service information for the exact pins and procedures.

After repairs, clear the code, run the applicable drive cycle or system self-test (varies by vehicle), and confirm the secondary air injection control circuit no longer indicates a low condition and the MIL remains off.