I have a 2002 obw with the p0420 code. It had very old gas in it. I've tried some of recommended fixes ie xylene, sea foam thru vacuum lines, good gas for multiple tanks. The monitors are fine except caty. How do I tell o2 sensor is bad, since the monitor for o2 is ready, does that's how a good sensor? What brand of sensor is recommended ? Should I replace back one first

It is almost never the sensor with that code.

 

Double-check for vacuum leaks and exhaust leaks.

Thanks dave. I've look for some and got under to look for exhaust. I'll look extra hard

Just because the sensor finishes the monitor checks doesn't mean its actually good. The range of acceptability for a good sensor output is a lot wider than the range they set for the effiency check. Basically for the sensor, the heater has to work, the sensor has to start making voltage within X amount of time, and the voltage output has to be within the range of the sensor. The voltage varies slightly with different sensor designs, but the gyst is the same.

The catalyst monitor has to use those sensor outputs. It compares output from the front sensors with output from the rear sensors, and uses those to determine if the cats are functioning properly.

 

Subarus are finicky about this, Subaru doesn't set a really wide range for the difference between the sensor outputs. So if one sensor is slightly out of range, it can set off the P0420 code, without setting a code for the sensor.

 

Sometimes a new sensor is the cure. Sometimes you need a new cat.

Most of the time its something like an exhaust leak, or vacuum leak, or dirty injectors, something that causes the fuel mixture to be too rich or lean, or one of the O2 sensors to give a false reading.

I hear a faulty PCV valve can even cause it...

What is a good way to test for leaks. Got some insense today to try tmr. Replaced some vacuum lines and another bottle of sea foam down the egr and other intake lines. Would oxygen sensors from my 98 obw work ?

Thanks guys. Cardoc helped me. I had the p0420

Changed wires plugs with Ngk and sea foam in tank and intake. A new fuel filter fixed it.