# P0491: Secondary Air Injection System Insufficient Flow Bank 1 **Verdict:** Safe to drive Emissions - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $50-$1,500 - **Urgency:** Keep driving - **Most likely:** Failed air pump or stuck check valve - **DIY?:** Steps 1-2 ## Quick answer P0491 means the engine computer has found that too little fresh air is reaching the bank 1 exhaust from the secondary air injection system. This system pumps air into the exhaust for a minute or two after a cold start to heat the catalytic converter faster. A failed air pump, a stuck check valve, or blocked air passages are the usual causes. ## What does P0491 mean? Secondary air injection, sometimes called the air pump system, runs only for a short time after a cold start. An electric pump pushes outside air through a hose and a one-way check valve, sometimes called a combination valve, into the exhaust ports or manifold. The extra oxygen burns leftover fuel in the exhaust, which heats the catalytic converter quickly and cuts emissions during warm-up. Bank 1 is the side of the engine with cylinder 1. While the pump runs, the engine computer checks the result. Most systems expect the upstream oxygen sensor on bank 1 to read very lean, while some use a pressure sensor in the air line instead. If the change is smaller than expected, it sets P0491. The test only runs after a true cold start, often once the engine has sat for hours, so the code can take days to appear or to clear. V engines use a separate code for the bank 2 side. ## What are the symptoms of P0491? - Check engine light on, usually with normal driving - Loud whine, roar, or grinding from the air pump after cold starts, or no pump sound at all - Rough or uneven idle right after a cold start on some vehicles - Failed emissions test or a secondary air monitor that will not complete - Moisture, soot, or corrosion around the pump, hose, or check valve ## What causes P0491? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failed secondary air pump** (most common). The pump sits idle almost all the time, and moisture can collect inside it, especially if a leaking check valve lets exhaust and condensation back through. Rust and seized bearings follow, and the pump may blow a fuse, grind loudly, or go silent. Short trips and humid or salted roads tend to speed up failure. 2. **Stuck or leaking check valve** (common). The one-way check valve, or combination valve, should open only when the pump runs and stay shut against hot exhaust the rest of the time. Carbon buildup can jam it shut, blocking air, or hold it partly open so exhaust and water flow back toward the pump. A stuck valve often ruins the pump it was meant to protect. 3. **Clogged air passages in the head or manifold** (common). On some engines the injected air travels through small passages cast into the cylinder head or exhaust manifold. Carbon from the exhaust gradually narrows them until little air gets through, even with a healthy pump. This is a known weak point on certain engines, and cleaning the passages can be labor-intensive. 4. **Failed relay, fuse, or wiring** (less common). The pump draws high current through a relay and fuse, and both can fail. A corroded relay socket or a broken power or ground wire leaves a good pump with no power. If you never hear the pump on a cold start, check the fuse and relay first, since they are inexpensive and easy to test. 5. **Vacuum control or air hose problem** (less common). Many systems open the check valve with engine vacuum switched by a small solenoid. A cracked vacuum line, a failed solenoid, or a split or disconnected air hose means air either never reaches the exhaust or escapes on the way. These parts are cheap, but finding them takes a visual check and a hand vacuum pump test. ## How do you diagnose P0491? 1. Listen for the pump during the first minute after a cold start, or command it on with a scan tool. 2. Check the pump fuse and relay, and measure voltage at the pump while it is commanded on. 3. Watch the bank 1 upstream oxygen sensor while the pump runs. It should swing strongly lean. 4. Remove the hose at the check valve and look for water, carbon, or soot, which mean the valve leaks back. 5. Test the vacuum line and control solenoid, and check the air passages if the pump and valve test good. ## How do you fix P0491, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace relay, fuse, or vacuum line | $50-$200 | Cheap parts worth checking first. | | Replace check (combination) valve | $150-$500 | Often replaced together with the pump if water reached it. | | Replace secondary air pump | $400-$1,200 | Pump location and part price vary widely by vehicle. | | Clean carbon from air passages | $300-$1,500 | Some engines need the intake or manifold removed for access. | ## Is it safe to drive with P0491? > **Safe to drive.** Yes, P0491 is generally safe to drive with, because the system only matters for the first minutes after a cold start. Fix it before an emissions test. If the pump is very noisy, smells hot, or keeps blowing a fuse, have it checked soon to avoid electrical damage. ## Will P0491 fail an emissions test? P0491 fails an OBD-based emissions test in most states while the check engine light is on. The secondary air monitor only runs after a qualifying cold start, so after a repair it may stay not ready until the car has sat overnight and then been driven. Plan a few days of normal use before testing. ## After the repair: clearing P0491 After the repair, clear the code and let the car sit for several hours, ideally overnight, so the engine is truly cold. Start it and let it idle, and you should hear the pump run briefly. The fix worked when the secondary air monitor shows complete and P0491 does not return after several cold starts. ## How to prevent P0491 - Replace a leaking check valve early, before exhaust moisture reaches and ruins the pump. - Listen for the pump's short run after cold starts, and investigate if it goes quiet. - Keep the pump intake filter or hose clear of debris on systems that have one. - Fix misfires and oil burning, which add carbon that can clog the air passages. - Take a longer drive regularly so moisture in the exhaust system can fully dry out. ## Codes often seen with P0491 - [P0410](/codes/p0410): Secondary Air Injection System "A" - [P0411](/codes/p0411): Secondary Air Injection System "A" Incorrect Flow Detected - [P0171](/codes/p0171): System Too Lean (Bank 1) - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected ## Frequently asked questions ### Can I drive with a P0491 code? Yes, P0491 rarely affects how the car drives, because the secondary air system only works for a minute or two after a cold start. Emissions are higher during warm-up, and you will likely fail inspection. Get it checked soon if the pump is very loud, smells hot, or blows fuses. ### What does secondary air injection do? It pumps fresh air into the exhaust right after a cold start. The extra oxygen burns leftover fuel in the exhaust, which heats the catalytic converter quickly so it starts cleaning the exhaust sooner. The system shuts off once the engine and converter are warm, usually within a couple of minutes. ### How much does it cost to fix P0491? Most P0491 repairs cost $50 to $1,500 at an independent shop. A relay, fuse, or vacuum line runs $50 to $200, a check valve $150 to $500, and an air pump $400 to $1,200. Cleaning clogged air passages varies widely, from about $300 to $1,500. ### What is the difference between P0491 and P0410? P0410 is a general fault in the secondary air injection system, often electrical. P0491 is more specific: the system runs, but too little air reaches the bank 1 exhaust. On V engines, a separate code covers bank 2. The pump and check valve are common causes of both codes. ### How do I know if my secondary air pump is bad? Listen at a cold start: a working pump makes a steady whir or hum for about a minute. Silence can mean a dead pump, fuse, or relay, while grinding or screeching suggests worn bearings. A shop can command the pump with a scan tool and check its voltage and airflow. ### Can I remove or disable the secondary air system? No, removing or disabling it is generally illegal under federal anti-tampering rules, and the car will keep failing OBD-based tests. Software that hides the code can also count as tampering. Repairing the system is the proper fix and the only one that reliably passes inspection. ### Why does P0491 only show up in winter? Cold, damp weather is harder on the system. Water trapped in the pump or check valve can freeze or corrode parts, and a weak pump may only fall short on the coldest starts. A code that comes and goes with the seasons still points to a part that is wearing out. ### Can a bad check valve cause P0491? Yes, a check valve stuck closed blocks the air, which sets P0491. One stuck partly open can let hot exhaust and water flow back toward the pump and slowly ruin it. Soot or water in the hose between the pump and the valve is a strong sign the valve has failed. ### Can I fix P0491 myself? Sometimes. Checking the fuse and relay, listening for the pump, and inspecting hoses are easy steps. On many cars the pump and check valve unbolt with basic tools, though some sit low or behind other parts. Cleaning clogged air passages in the cylinder head is usually a shop job. ### Will P0491 damage my catalytic converter? Not directly. Without secondary air, the converter simply takes longer to warm up, so emissions are higher for the first minutes of each drive. The bigger risk is an underlying problem, such as misfires that also clog the air passages, which can harm the converter if ignored. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other emissions codes - [P0443 Evaporative Emission System Purge Control Valve "A" Circuit](https://diagxcar.com/codes/p0443) - [P0446 Evaporative Emission System Vent Control Circuit](https://diagxcar.com/codes/p0446) - [P0449 Evaporative Emission System Vent Valve/Solenoid Circuit](https://diagxcar.com/codes/p0449) - [P0451 Evaporative Emission System Pressure Sensor/Switch "A" Circuit Range/Performance](https://diagxcar.com/codes/p0451) - [P0452 Evaporative Emission System Pressure Sensor/Switch "A" Circuit Low](https://diagxcar.com/codes/p0452) - [P0455 Evaporative Emission System Leak Detected (large leak)](https://diagxcar.com/codes/p0455) - [P0456 Evaporative Emission System Leak Detected (very small leak)](https://diagxcar.com/codes/p0456) - [P0457 Evaporative Emission System Leak Detected (fuel cap loose/off)](https://diagxcar.com/codes/p0457) - [P0496 Evaporative Emission System High Purge Flow](https://diagxcar.com/codes/p0496) - [P0498 Evaporative Emission System Vent Valve Control Circuit Low](https://diagxcar.com/codes/p0498) ## Symptoms - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) --- Source page: https://diagxcar.com/codes/p0491 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.