If you encounter a 2011-2012 Subaru Legacy/Outback or Tribeca with code P0128 (coolant thermostat/engine coolant temperature below thermostat regulating temperature), it may be caused by a very slight variation in coolant temperature between the left and right cylinder banks.
The slight variations can be contributed to owner driving patterns. As an example, how long a vehicle is permitted to warm up, heater use during cold or hot engine temperatures, etc. An air pocket within the coolant system can also cause the code to set.
A countermeasure heater hose pipe is now available which contains an orifice, designed to regulate coolant flow past the coolant temperature sensor.
The replacement heater hose pipe is available under P/N 14070AA361 (the old P/N is 14070AA360).