Emission controls · Powertrain · generic (SAE-defined)
P0491: Secondary Air Injection System Insufficient Flow (Bank 1)
P0491 means the secondary air injection system is not supplying enough air to the Bank 1 exhaust, the side with cylinder 1, during its cold-start operation.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0491 sets
After a cold start, the powertrain control module (PCM) runs the secondary air pump and opens the Bank 1 air valve, expecting the Bank 1 upstream oxygen sensor to read lean or a pressure sensor to show flow. If the response is weaker than expected, the PCM sets P0491. Test details and timing are manufacturer-specific.
Common causes of P0491
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Weak or failed secondary air pump
- Bank 1 check or combination valve stuck closed
- Carbon-clogged air passages to the Bank 1 exhaust ports
- Cracked or disconnected air hoses
- Failed air pump relay or fuse
- Slow Bank 1 upstream oxygen sensor
P0491 symptoms
- Check engine light on
- Unusual pump noise at cold start, or no pump noise
- Usually no drivability symptoms
How P0491 is diagnosed
- Check for related secondary air and oxygen sensor codes
- Listen for pump operation after a cold start
- Verify the fuse, relay, and power to the pump
- Inspect hoses and the Bank 1 check valve for cracks, moisture, or soot
- Command the system with a scan tool and watch the Bank 1 upstream sensor for a lean response
- Inspect passages for carbon buildup where accessible
Typical fixes for P0491
- Replace the secondary air pump
- Replace the Bank 1 check or combination valve
- Clean blocked air passages
- Repair hoses or replace the relay
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Manufacturer notes
Commonly reported on some Volkswagen, Audi, BMW, Mercedes-Benz, and Toyota engines. Definitions can vary by manufacturer; check factory service information.
Related codes & warning lights
P0491 FAQ
What does code P0491 mean?
P0491 means the secondary air injection system is not supplying enough air to the Bank 1 exhaust, the side with cylinder 1, during its cold-start operation. The standard definition is "Secondary Air Injection System Insufficient Flow (Bank 1)."
Can I drive with a P0491 code?
Yes, in most cases. The system only works briefly after cold starts, so drivability is normally unaffected, but the vehicle will likely fail an emissions inspection.
Do all cars have secondary air injection?
No. It is used on certain engines to meet cold-start emissions targets, and many vehicles do not have it.
Why are carbon deposits a problem for this system?
Air is injected through small passages near the exhaust ports, and over time carbon can restrict or block them, reducing flow even when the pump works.
Keep researching
More emission controls codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.