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Code 420 - Catalyst Inefficiency

09-07-2009, 06:13 PM
Is there a way to determine for sure if this is a bad cat vs bad oxygen sensor(s)? The car is a 2000 V8 w/ 95K miles on it. IF it were an 02 sensor would it have to be one of the rear ones for this code?

I'm hoping it isn't the cats but fear it might be. I'd love to just replace the 02 sensors and have the problem fixed but I also don't want to spend the $100 on sensors and not have that fix the problem and still have to replace the cats. I recently got the car so I do not know if the sensors have been replaced in the past. If they have not they probably need it anyway but just looking for others input/experiences.


09-07-2009, 09:07 PM
An exhaust shop could likely install backpressure sensor in place of the O2 sensors to determine accurately if the problem is the converter or the HO2S. Keep in mind though that converters rarely fail under normal conditions. If the converter actually is the problem, make sure that any vacuum leaks or misfires are repaired before replacing the converter. If not, the new converter will fail prematurely as well.

Also, you may be able to replace both Bank 1 O2 sensors for less than the cost of the backpressure test (I have no idea what the test would cost) and it would not be wasted money. With 95k on the car, if the sensors have never been changed, they are about due anyway, and could be reused with a new converter.

Per the factory service manual, the possible causes for this Bank 1 code are:

Use of leaded fuel
Damaged HO2S
Malfunctioning ECT
High fuel pressure
Damaged exhaust manifold
Damaged catalytic converter
Oil contamination
Cylinder misfiring
Downstream HO2S wires improperly connected
Damaged exhaust system pipe
Damaged muffler/tailpipe assembly
Retarded spark timing


09-08-2009, 07:51 AM
Sometimes with a bad cat you will hear a rattle where as the material has broken lose.

09-09-2009, 08:07 PM

99.9% Bank 1(passenger side) catalytic converter is bad!

Tap the catalytic converter with a tool and compare bank1 CAT with bank2.

A blown catalytic converter will sound like an empty tin can.

By using a compression testor on the down stream, O2 sensor spark plug hole on bank 1, there would
be no back pressure for catalyst.

No pressure = Blown cat
High pressure = Plugged cat

The down stream O2 sensor is a feedback signal, it tells the ECU if the catalytic converter is working, if not, the ECU will throw codes P0420 (bank1) or P0430 (bank2).

12-08-2009, 09:30 PM
im glad some one posted on this subject.... tonight I was driving my lincoln ls 2002 v8. I was getting on it and when I went from 1-2 gear I noticed the cleck engine light pop on... pulled the code same thing 420.. now When i bought the thing it had 4 bad coil packs and who knows how long the prevous owner was driving with the bad coil packs.... im pretty much figuring the cat is pluged up. im going to let it idle in the garage and shoot the temps of each cat and compare.. which ever one it hotter is the plugged one and if ones hollow.. ill replace that one. anyone know how much a cat is? im going to see about ebay tonight

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