My 2008 mercury mariner turns the key but won't start my car

Hello, Do you hear the normal sound of the starter turning with the key turned to the 'start' position? If not, you may have a weak battery, bad starter relay, or bad starter motor. If you hear rapid clicking, that is likely a bad or dead battery. If you hear nothing at all, you may have an issue with the starter relay, but the ignition switch, which should be tested last, may have become faulty as well. Start by checking the battery, even though the radio and lights work, and ensure it is charged. Afterwards, check the battery terminals, starter relay, fusible links, then the starter. If you prefer, a starting and charging system test can diagnose the issue fairly quickly. Let me know if I can be of more help.