Code Definition

P0491 — Secondary air injection system low flow Bank one

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About P0491

P0491 is a generic (obd-ii standard) powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0491 is stored, the module has detected a signal or supply voltage lower than the expected operating range.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0491 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The Powertrain Control Module (PCM) sets this code when it detects insufficient airflow in the secondary air injection system on Bank 1. This system introduces fresh air into the exhaust to help reduce emissions during cold starts. Low airflow may be caused by a failing pump, blocked passages, or a malfunctioning control valve.

Quick Facts