[thstone]

There is no sensor (per se) that tells the air pump when to run. Instead, the air pump is commanded on by the ECU depending on coolant temperature (for cold start operation) and engine rpm (for pressure test at idle).

Use your Durametric to manually turn on the SAI pump. That should verify that the pump and all of the electrical connections are good. And if the pump operates manually, then it should operate properly when commanded by the ECU (whether cold start or pressure test).

The most common problems are hose leaks and inop valves.

What CEL code are you getting? That will help us point you in the right direction.