2002 Sienna cat

I’m looking at an 02 Sienna for sale. Have seen the online ad but have not seen the van in person yet. Owner says CEL is on due to cat problem. Asking price is about $1500 below market, I assume because owner thinks cat needs replacing.

Does anyone have experience with this model/yr and can tell me what to look for re the cat issue? I’m assuming worst case scenario is that I have to replace the cat for $1500 or so. Are there any typical problems with this engine (coolant leak?) that would cause cat to go bad? Or is it more likely that it’s just a bad O2 sensor?

PS: 150,000 miles on the van.

An illuminated CEL is a very bad sign. I’d walk away from this vehicle.

The problem with a CEL light is that all you know is something is bad.  Maybe the owner is right, or maybe the car has two problems and the other one, which you don't know about because it only has one light to light, and the car may need a new converter to go along with a new engine, that you don't know about because you think the only problem is the converter. 

I would pass on this one.