Frontier engine light issues

[sh5585]

I'm looking to buy a '99 Frontier V6. The problem is the check engine light on the truck won't go out. The codes are for all four O2 sensors and the knock sensor. The guy selling the truck replaced all of the O2 sensors and counless other emissions pieces to try and fix the problem (to no avail). He has all of the papework, so it seems to check out.

He did not change any of the cats. I'm wondering if four bad O2 sensors means that cats are bad? The truck has a ton of miles on it, but the engine and frame are in good condition and the price is pretty good.

Anyone have any ideas or a general direction to point me? I don't want to get hosed if I buy the thing! Thanks for the help!

[Towncivilian]

NTB99-004B - 1996-00 PATHFINDER, 1999-2004 FRONTIER (V-6) AND 2000-2004 XTERRA (V-6); MIL 'ON' WITH O2 SENSOR DTCs

[sh5585]

Thanks for the info! Just printed it out, gonna give it a shot!