My check engine light came on. the code is the following

P138 M 1/2 O2 Sensor Shorted To Voltage Oxygen sensor input voltage maintained above normal operating range.

I also noticed when I start it for a few minutes the exhaust fumes are quite black and then after it warms up it goes back to normal. I also get 11.7 miles / gallon, not sure if it's in the normal range as it is my first V8 engine.

Thanks for any advice.

 

You need exhaust work. When mine did that (same code, also P0420), I put in a whole new exhaust, but I wanted a performance exhaust anyway. The cheap fix is probably swap out the cat conv and both O2 sensors.