What causes a P0491 code?

P0491 can be triggered by several issues — see the verified probable causes and step-by-step diagnostics shared by mechanics and owners on OBDIY.

Can I drive with a P0491 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P0491 active.

Follow the step-by-step diagnostic on this page, then read the verified fixes contributed by other Heavy Truck owners — the answer marked "Verified Cause" worked for the original poster.

OBDIY

OBD-II

P0491

Secondary Air Injection System Insufficient Flow Bank 1

What it means

Indicates the ECU has detected the secondary air injection system is not supplying enough fresh air to the exhaust stream to aid catalyst light-off or emission reduction.