Vehicle: 2002 Dodge Caravan SE (3.3L V6)

The vehicle has had a check engine light for several months now and i've decided to finally address it. The DTC code is:

P0171 Generic
System Too Lean (Bank 1)

Symptoms

* on cold start then allowing the car to idle the vehicle's RPMs can drop below 500 RPMs (not always). The experience is a rough idle and the car has even shut off.

* on cold start when stepping on the gas to accelerate the car may not respond as if it had no "umph" / very weak. It eventually kicks in and drives properly.

Note: Almost 2 years ago i replaced the UpStream Oxygen Sensor on the manifold.

My first instinct is to simply replace the Downstream Oxygen Sensor which is attached to the Cat Converter but before i bark up the wrong tree WHAT STEPS WOULD YOU TAKE to identify / resolve this issue? If left to my own devices i typically waste time and money heading down the wrong direction before doing the most obvious thing.

I'm still educating myself but if this issue is with the downstream O2 sensor is the problem more likely a dying cat converter?

Much appreciated.