P0018 doesnt come up in my GM Service information for the 2012 Sonic. It is a a cam actuator/solenoid valve code - but are you sure it was a P0018? Maybe P0016? Either way, its likely a bad cam actuator (sprocket). Depending on the actual code its rather intake or exhaust side. Any work done to it recently?
Edit: P0016 is for the intake side, P0017 is for exhaust. The actual code descripion reads "intake/exhaust camshaft position not plausible.