It would be nice to know what kind of car and engine??P2181 is usually T-stat related. If you knew somone with a vag com the could scan it and look at OBD2 freeze frame data and see that p2181 fault was most likely set at a below normal engine operating temp. This indicating the t-stat is stuck/sticking open causing the engine temp to drop setting the fault. Replace T-stat. Green ECT are an updated sensor from the black ones.