Immobilizer is active, will not let me start the car

The way they test a key normally just tells that it is sending out a signal, not that the signal is the correct one that the vehicle in question uses. However most of the time it isn't a key issue. It is usually related to the parts of the immobilizer in the vehicle. The only reliable way to test them is with a factory level scan tool that can talk to the security system. With that you can track what the system is actually doing. For instance it could be that the immobilizer relay is bad, or that the transponder antenna isn't sending out the power pulse that queries the key. No real way to test that without the scan tool and a signal tester. There isn't even a unique fuse for it as it draws power from the body control module and the instrument cluster.