The car sat unknowingly with a dead battery for about a month. I removed and charged the battery. After reinstall it cranked and ran fine but the CEL is on and the immobilizer light is flashing. I tried to clear the codes but p603 won't clear and light stays on. I ran through the drive cycle and still won't. I have no idea about immobilizer light. I've got to get emissions done soon. What can I do?

 

Could be due to a low battery charge or battery cable connections. Clean off any corrosion on the terminals. You can get the battery checked at an auto parts store.