P0491 indicates that the powertrain control module has detected insufficient airflow in the secondary air injection system on bank 1 of the engine. The code is set when the measured flow does not meet the programmed threshold during startup or operation.
The pump may cease operation or produce insufficient pressure, preventing adequate airflow into the exhaust system.
Deposits or obstructions restrict airflow, causing the system to fall below the programmed flow threshold.
A bad relay or corroded connector can prevent the pump from receiving power, mimicking a pump failure.
Air leaks reduce system pressure and flow, triggering the insufficient flow detection.
In rare cases, a malfunctioning control module misreads sensor data and logs the code despite a functional pump.
No, replacing the PCM is seldom necessary; the code is usually caused by a failed pump, blocked passages, or electrical issues.
A severely clogged converter can affect exhaust flow, but it does not directly cause the air injection system to register insufficient flow.
After correcting the underlying cause, the vehicle should pass emissions testing, though a relearn procedure may be required.
Repair time varies with the specific cause, ranging from a few minutes for a relay replacement to several hours if the pump or passages need cleaning or replacement.