What causes issues with the secondary air injectors? on 2005 Isuzu Ascender

The check engine light stays on all the time and it will not pass smog. The mechanic said it is a problem with the secondary air injectors but he would need to do a 3-4 hour diagnostic test to find the cause and the repair could cost $300 to $400 dollars.

Asked by for the 2005 Isuzu Ascender
The most common issue is that the control valve won't close correctly and condensation water returns to the pump and damage it. However it could be bad control solenoid or broken, leaking hose also.