metrostop

96 lx450...new cat. front and rear. all o2 sensors,did drive cycle 3 times, still p0420 is coming on,any help..do I need to reflush the comp.?,

Jacket

You've done the right things. Mine went away after I replaced both O2 sensors. Did you unplug the battery for a bit to reset the code? Did you use OEM sensors?

98SNAKETR

What brand of Cats and O2's are you using?

PO420 codes are usually triggered when the upstream and downstream O2's have similar readings, giving the ECU the idea that the Cat(s) aren't functioning as they should.

That being said, the 1FZ isn't nearly as picky about this as most other modern vehicles

I'm running a gutted MagnaFlow Y-Pipe, gutted Bosal secondary cat and OEM O2's on my 97 TLC

Initially, I thought I would have to wire up some O2 simulators after gutting, but I've running like this for over 40,000 miles now and have yet to throw any codes

Z07

I would put the LX on a computer/phone/tablet via USB or Bluetooth to ODB II adapter and load up something like Torque and watch what the O2 sensors are actually doing. You should see the upstream S1B1 going rich./lean frequently and the downstream S2B1 doing the same but much more slowly and lower magnitude.

Also an induction leak after the MAF or an exhaust leak before the last O2 sensor will throw this code.