Check engine light is flashing when engine is load bearing

Engine is actually running great, especially after the tune up. The Check engine light has been on since we bought it from our former mechanic, who we had trusted when he said everything the car needed was still minor. He said the light was on because the middle pipe needed to be replaced, but it was nothing big, just made the car loud. I trusted the wrong person.

Now the Check engine light only flashes when the car is load bearing. No shaking, no rough engine. The tune up took care of the engine misfiring. I know that one of the O2 sensors needs to be replaced and I just wanted to know if that could be why the Check engine light flashes.