P0491 - what sensors detect the insufficient air flow

Question

pump runs about 10-20 seconds. replaced bad valve solenoid works still get code. What measurement determines insufficient flow?

Answer 1

P0491 Secondary air injection insufficient flow (Bank 1)

The air injection pump only run at cold start up and is on for a short period of time 10-20 seconds that correct. This is downstrean of the MAF (mass airflow) sensor , so what dtects a insufficient flow is system back pressure and the 02 sensor.Some of the common causes are the secondary air divertor valve , where its vacuum source is restriced or leaks.