2011 Legacy engine diesel's at low speed, while excellarating! 2.5 SOC

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Still has power at higher speeds. Legacy has 162xxx miles on it. Had a trouble code " P0026 ". Disconnected battery and cleaned all connections I could find! Trouble code went away, has not returned! PS. Engine light is not on! What seems to make the problem better or worse? All the time How long have you had this problem? A little while now