Yep. 0420 is Catalyst Efficiency out of range, 0131 is Oxygen Sensor reading low.

Likely causes:
A.) Bad Catalytic Converter (If it is this, get the engine checked over by a shop to make sure it's well tuned)
B.) Bad Oxygen sensor (It's not likely this, as the sensor has to die a very specific way for it to look like a bad cat and not a dead sensor)

Had those EXACT codes thrown for my 2002 Suzuki Aerio and it was the cat. Take it to a muffler/exhaust shop and have them look at it. It will not pass emissions as it is right now, so if you're looking at buying it, remember it's the seller's responsibility to provide a passed emissions certificate. You might wanna ask how long the CEL's been on and all, and consider if you're gonna buy it that you might not be able to tag it till you get that fixed, though it's not a huge deal.