I am getting a P0420 code. From the looks of things it is supposed to be either a downstream 02 sensor or bad catalytic converter. I am assuming the downstream 02 sensor is behind the cat. Are there 2 cats and 2 downstream 02 sensors. Just not sure how to verify what exactly is the problem. The car is a 99 with about 195,000 miles. Any help or ideas.
Thanks

 

There is one cat and one cat sensor.

The sensor might be bad. Or perhaps its working just fine, and telling you that the cat is bad.

 

Possible causes:

Faulty three-way catalyst converter bank 1
Exhaust tube
Intake air leaks **
Faulty oxygen (O2) sensor **
Faulty fuel injector(s)
Leaking fuel injector (s)
Faulty spark plugs **
Improper ignition timing
Faulty engine control module (ECM)
Dirty air filter **

(**) Check/replace these first-off-the-bat.

 

P0420 almost always means a bad cat. They are not expensive as long as you don't live in California

However, you should replace the sensors along with the cat.

 

Hopefully it's just a bad or lazy O2 sensor. When I threw that code, that's what it was. A new one fixed the problem. How many miles are on your O2 sensors?

 

But anyway, it can be the o2 sensor(s) right before the cat. Or the cat itself.

 

Observe the Law of Parsimony until it no longer applies ... "simpler solutions are more likely to be correct than complex ones."