If your check engine light is on after a dead battery, it's possible that the battery voltage drop may have triggered a fault code in the vehicle's computer. Here's how to clear the check engine light: 1. Check the battery connections: Make sure the battery connections are clean and tight before clearing the check engine light. 2. Use an OBD scanner: Connect an OBD scanner to the vehicle's diagnostic port and check for any stored fault codes. If the fault code was due to the dead battery, try clearing the code to turn off the check engine light. 3. Disconnect the battery: Disconnect the negative battery cable and wait for 15-20 minutes. This will reset the vehicle's computer and clear any stored fault codes. 4. Reconnect the battery: Reconnect the negative battery cable and start the vehicle. If the check engine light is still on, it may be an indication of a more serious issue that needs to be addressed.