CajunSubOwner

All: MANY thanks for the assessments and the recommendations. I will follow up and re-post with update - probably in a few weeks.

BLUF: Could failing/poorly functioning catalytic converter be related to/caused by previous gasket head cracks or by repair work done to replace cracked gasket head?? Mechanically challenged 08 Forester owner here. A few months ago, we found out that the gasket head was cracked. Right after we picked it up, we noticed that it seemed to have less power and, very obviously, the gas mileage was lower. The repair shop did not believe (or at least admit) that there was any connection between the replaced gasket head work and the substantial reduction (I calculated it to be about 3-5 MPG difference) in fuel efficiency. Three months after the repairs, the check engine lights came on. The repair shop advised that the sensors showed that the catalytic converter was failing/had failed. They said there was little difference in temperatures going in and going out of the catalytic converter system (I'm simplifying what they said but that sums it up). I asked about whether it could be caused by the engine not "burning" all of the gas, but they said that if that were the case the error codes would be different. And that I need a new catalytic converter. I'm hesitant to shell out the cost without knowing why it failed (so that it doesn't fail again and I'm looking at buying another one. Is there a way to determine why the CC failed? Should I take it for a second opinion? What other questions should I ask? Many thanks in advance for your help.