engine light was on. so I took my car to repair shop. Mechanic claimed that he changed the cat and he told me to drive 3o miles and then go to smog shop. After driving around 32 miles, there was no engine light. But when smog shop guy check my car with small computer device before starting the actual smog; it showed cat converter code again. So i took my car back to repair shop and told whatever code it was showing. This mechanic connect this device to car and says that it is not showing any code on this device because he changed it. He claims that smog guy shop maybe forgot to delete those codes from previous car (whatever he tried to smog before). Moreover, he also claimed that these are history codes meaning those things were wrong before but now they are fixed. I don't know anything about cars. I feel like mechanic just took my money but actually didn't change any catalytic converter. Can someone tell me what is really wrong ?