If you encounter a Subaru owner's complaint about an idle fluctuation during cold starts which generally last about 20 to 30 seconds, the cause may involve a combination of factors, including fuel blends, percentage of Ethanol content in the fuel, temperature and narrow operating range of the air fuel ratio sensor. This applies to all 2005 and later naturally aspirated (non-turbo) PZEV (partial zero emissions) models.
The main cause of this idle hunting is fuel related and likely not a vehicle fault.
PZEV engines are designed to run on fuel blended for distribution within certain areas of the U.S. (such as the state of California). Fuels available in other markets, that may have adopted California emission standards may be a slightly different blend. If the vehicle is operated in any other state than California, the vehicle will operate on gasoline meeting Federal specifications. However, PZEV vehicles may experience this issue especially during the summer months when summer blend fuels are distributed. There is no affect on vehicle emissions, and no short or long term damage will occur due to this issue.
The condition may be reduced or eliminated by changing to a fuel that contains a lesser percentage of (or no) Ethanol. Also, a light throttle application while the vehicle is exhibiting this condition will stabilize the idle. As we move towards cooler weather, many areas of the country will be changing to a winter blend of fuel which may also alleviate the condition.
If this condition is confirmed, no repair attempts should be performed beyond confirmation and checking that all other systems are operating within design parameters.