I can't get to the OBD connector on my car at the moment.
As far as I know the OBD II voltage is active even when the key is in lock.
Can anyone confirm this?
edit : It's for a product that will be powered from the OBD II connector.
Pin 16 on the ODB-II connector is specified as "Battery Power" or "Battery Positive". There will always be 12v power between pin 4 and 16. Note that pin 5 has "ground" in the description, but that is just for the communication signalling and not for power.
The connector will always have power on these two pins regardless of the key position, and when you plug a device into the port it will have power and may come on by itself (many scanners do this).
However, this is only power and you won't be able to communicate with the car. The car's computers will not have constant power and are typically only active when the key is turned to the ON position. Some cars may power up certain CAN components or even the ECU in the ACC key position, but I've never seen that happen (limited experience).
You didn't specify what car you are trying to diagnose, but in my experience vehicles need to be 'on' in order for the OBDII to read.