"Could it be the cooling fan. How can I test if the cooling fan is running properly ?"
Does the fan come on when it hits L? If it does, then you know it is working properly.
"My mechanic told me that "if the needle does'nt go beyond L to the red area, cooling fan is alright". Is that true ?"
I4 Zetecs do run a little hotter than the V6 Duratecs. Technically, if it is in the NORMAL range, it is considered acceptable.
