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chrysler pacifica po 420 code

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2 replies to this topic

#1 bobby c

bobby c
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  • 5 posts

Posted February 23, 2011 at 08:50 am

2 years ago 420 code dealer replaced cat. Last year 420 code 2x to dealer replaced 4 wire harness from cat o2 sensor.Last week check engine light 420 code again. Any ideas???

#2 AllanC

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Posted February 23, 2011 at 08:25 pm

Code P0420 indicates that the catalytic converter has dropped below a minimum threshold for efficiency and is no longer reducing exhaust pollutants to levels prescribed by federal law. The engine controller software compares the switching rates of the upstream and downstream oxygen sensor. So if the switching rate of the downstream sensor approaches and looks like the upstream oxygen sensor signal, then it is assumed that the catalytic converter is no longer functioning properly and the vehicle is no longer within federal emission guidelines and code P0420 is set.

Since the catalytic converter was recently replaced then it should last longer than 2 years before losing effectiveness in cleaning the exhaust. It is possible that some operating condition is causing the catalytic converter to fail prematurely. A misfire condition in 1 or more engine cylinders can dump raw gasoline into the converter and cause it to overheat and fail. A leaky exhaust valve can allow the air fuel mixture to leak into the exhaust stream during the compression stroke on an individual cylinder and cause the catalytic converter to overheat. An engine that burns excessive amounts of oil or a leaky head gasket that allows coolant into the combustion chamber and into the exhaust can fail a catalytic converter and/or oxygen sensor. An exhaust manifold leak can allow extra air to enter the exhaust and fool the oxygen sensor to send an erroneous signal to the engine controller.

Now none of these conditions are necessarily easy to fix. An engine misfire or failed oxygen sensor should set a unique diagnostic code other than P0420. A visual inspection of the exhaust system and checking of the manifold bolts for tightness should be performed. The wiring between the oxygen sensors, upstream and downstream, and the engine controller should be checked. The electrical plugs at the engine controller should be disconnected, inspected and cleaned to make sure no excessive resistance has occurred within the circuitry. A scan tool should be used to check the switching rates of the upstream and downstream oxygen sensors. It is possible that a lazy sensor or one that is slow in responding to changes in the air fuel mixture can generate erroneous signals and set false diagnostic codes. Both oxygen sensors should be removed and visually inspected. A brown or gray color should exist on the probe. Other colorations could indicate foreign substance(s) such as oil, coolant, etc are entering the combustion stream somewhere.


#3 B10alia

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  • 1,696 posts

Posted February 25, 2011 at 09:56 am

How many miles are on the car? How does it drive? How does the temperature gauge ride under normal conditions? Does it make any unusual noises? Any information you have could hepl pinpoint exactly what is going wrong.
Just my 2 cents, if the downstream sensor has failed or is failing, the computer could get the wrong message as to what is going on inside the cat. This might not set a code; O2 sensors can fail partially and cause problems without setting a code. My car only has 1 O2 sensor, so this next bit is a complete guess, but the downstream O2 sensor would not cause driveability problems because it's not used to adjust the mixture. Somebody please correct me if I'm wrong on that, but I would lean toward the downstream sensor itself being the problem.

Edited by B10alia, February 25, 2011 at 10:03 am.


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