New and inexperienced here so please be gentle. I have a 2006 Scion tC with 139k miles. Check engine light came on a few weeks ago and ran code. P0420 - Catalyst System Efficiency Below Threshold Bank 1 Center up. Cleared codes and continued to run car.
Boyfriend used a code reader on the car last night and left it hooked up. Would not start this morning until I removed device which he forgot. Car has minimal power and seems to struggle...even idling extremely low when at a stop light or sign which I had to hold my
foot on brake and gas at same time to prevent car from stalling. Check engine light is not on. Took car to Auto Zone. Alternator is fine and battery voltage is 12.66v and measures 458 out of 575 which indicates charge required. I had a similar issue years ago and had
fuel pump replaced and all my sensors. I do have one out of the two radiator fans working properly which apparently has been this way since I bought the car used 4 years ago. It does tend to go through a lot of coolant though (replenish fluid every 3 months maybe).

I was told that due to some oil by engine I may have a head gasket leak.

Any suggestions on what to do?