I have an '03 Duramax and leaking coolant. I have been putting coolant in all the time. The leak seems to be from the water cooler and need to know if there is a gasket needed for it when replaced, because I was told by the parts counter person that there is just an O-ring. Is this true? Help!!

welp, are you talking about the cap? Im kind lost as to what you are asking. have you checked for leaks yourself. check over all the coolent lines to see if there are any cracks.

I think you are talking about the oil cooler. Where exatly is it leaking from? Drivers side or Passenger?

Get under the truck, if the leak is on the drivers side front of engine it's the water pump.

Thank you everybody, indeed it was the water pump. Was a pain to get to it but got it done.