my check engine light is on what is the problem? on 1996 Toyota Land Cruiser

i used a hand held diagnostic machine to check it and the code that came back was PO420. please get back to me asap

Asked by for the 1996 Toyota Land Cruiser
Use a scanner that can provide you with rear O2 sensor readings. If you have a smart phone there is a wonderfull new piece of equipment for $99.99 that can provide you with this info. Go to to see. I really can't believe how good the device is. Of course it comes from silicon valley.
The truck is 16 years old. You most likely have to replace the cats.
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P0420 is low catalytic converter efficiency.Could be a bad cat, false O2 sensor readings or have failed due to an out of tune engine/excess fuel or misfire.
thanks again,what are you suggestion first?