I have a 2004 GMC Seirra 3500 and i'm always getting messages like coolant low or battery not charging then one day I stopped at a red light and oil pressure low message came on. I called a mechanic and he said change the oil pressure sending unit,so i did. No change. when it drops below 1000 rpm's it starts dinging. Any suggestions before I take it to a dealer and pay them a small fortune. Thanks for any help.