Check Engine Light

I have a 2003 Passat 1.8T. It has about 68,000 miles on it and recently the MIL light continues to stay on. The error code says engine cooling performence error. I have taken it to the dealership 3 times to no avail. It is not the engine coolant sensor, and my thermostat tested fine. Any solutions???:banghead:

What does the dealer say is the problem??