Just like most air injection systems, the O2 sensor is the monitor that see if the system is working.

When the engine is warmed up & the O2 sensor is in closed loop (controlling fuel mixture),
The engine computer will engage the air pump. At the same time open the diverter valve and then watch the O2 sensor for very low voltage output.

Something in that air injection CHAIN must be missing.
Does the pump ever run?
Is the O2 sensor "fresh"?
Are the vacuum lines from the switch-over valve to the diverter valve in good condition?

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