P0420 - Catalyst System Efficiency Below Threshold Bank 1

[TexasStig]

My 99 is giving me a P0420. Assuming it's not just looking to get high, Mr. Internet says:

A code P0420 may mean that one or more of the following has happened:

• A damaged or failed oxygen sensor (HO2S)
• Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings
• Leaking fuel injector or high fuel pressure
• Cylinder misfire
• Oil contamination

Car drives fine and doesn't appear to have any exhaust leaks.

The pointy board mentioned using this, as I do have a high-flow CAT & exhaust:
http://www.bigdaddiesgarage.com/mini-cat-cel-fix.html
Any suggestions on where to start? I'm due for an oil change anyway, so that's low hanging fruit. Mmmmm... munchies.

[motorollow]

Gabkwong is the master of NB CEL codes. Maybe he will chime in.

[EricJ]

Check the O2 sensor voltages after warm up at idle. The front one should be bouncing between .1 and .8, the rear should stay low, .1 ish.
If either sensor is reading 0 volts, it's either the wiring or the sensor.
If the front one isn't bouncing, either staying hi or staying low, its either the O2 sensor or one of the upstream sensors. The ECU should bounce between lean and rich at idle.
If the rear sensor is bouncing, i.e. following the front sensor, then it's the Cat.

You can measure the O2 voltages with an ODB-II reader. I use Torque on my Android phone and a Kiwi bluetooth adapter.

[TexasStig]

Check the O2 sensor voltages after warm up at idle. The front one should be bouncing between .1 and .8, the rear should stay low, .1 ish.
If either sensor is reading 0 volts, it's either the wiring or the sensor.
If the front one isn't bouncing, either staying hi or staying low, its either the O2 sensor or one of the upstream sensors. The ECU should bounce between lean and rich at idle.
If the rear sensor is bouncing, i.e. following the front sensor, then it's the Cat.

You can measure the O2 voltages with an ODB-II reader. I use Torque on my Android phone and a Kiwi bluetooth adapter.

Great; thanks! I hope it warms up enough in the next few days to give this a try.

[OZMDD]

You can use my scangauge if you need it.

[VFR750]

My 99 has had that code pop up for years now. A new front O2 sensor and spark plugs helped. The main thing that keeps it turned off is running it down the highway...........if I drive it in town more than continued higher RPM operation I can count on P0420 making an apperance, kinda points to weak cat function to me.