Just got this alarm on my stock 2006 STI. After some research I found the following:
A code P0021 refers to the VVT (variable valve timing) or VCT (variable camshaft timing) components and the car's PCM (powertrain control module, also called an ECM).
Most likely a P0021 DTC will result in one of the following: hard starting, poor idle, and/or stalling.
I have none of these symptoms. The car runs as well as it always had. Any ideas??