Ever since replacing the head gasket nothing but trouble. Now there is secondary air injection code P0410 and the cat readiness monitor flashing. I was able to reset them 3 times but this time the cat rediness monitor has been flashing over 1000 miles. Also just replaced downstream O2 sensor. The upstream O2 sensor is less than 1 year old. Any ideas? I want to replace the cat. Any recommendations, stock or universal? Car has been driven over 210k miles. Thanks..