Hey Everyone!

So I know enough about vehicles to be dangerous, I have the repair manual, code reader and the tools to fix stuff. I encountered the code P0161 and I also got a second one that is also P0161 but it has this "pd" thing after it on my code reader.
I was wondering could anyone here help me out on finding out what the code actually stands for? Everything in my exhaust system has remained the same since I bought the truck in 2015 at 100k. Do I need to change converters? Or something else? Or is it really just the O2 sensors that have gone bad. Would really appreciate the help!

Little back ground on vehicle: My truck is at 172k and I put about 2000 miles on it a month for work. So far I've had to replace the Alternator, Starter and battery, that was in November of 2016 when I was at around 155k miles. Nothing else has been done on the vehicle besides oil changes twice a month.