To properly repair your fault which is "cooling system malfunction" you need to replace your thermostat. You will need to replace the seal and plastic adapter/housing as well. These parts are pretty cheap.
When you replaced the sensor, the old one probably had a black top. The sensor you put in SHOULD of been green. If it wasn't, you need to go to a VW dealer and purchase a new ECT with a green-top, the sensors have been superseeded.
Now when you say you have idling issues, what exactly happens? Because in your description you said it happens in 2nd gear, which is impossible because idling is when the car is stopped and the engine is still running. Once you touch the accelerator pedal or the wheels are moving, you are no longer idling.
Replace the thermostat and install a green top ECT and the fault will be fixed.
Wednesday, March 27th, 2013 AT 10:44 PM