2000 Lincoln Ls V8 still getting code PO420 after replaceing Cat Conv,changed 02 sensors, still getting code PO420 and now when exiting the freeway the car turns off.... have to put it in park to restart and its fine, this has happened like 5 times already???Pleeease help? i really need info as to exactly what is wrong, what to do.... i'd really appreciate your help....:)

O2 sensors are after the cat converter and if you have 2 cat converters they will be after each converter
have you had the fault codes read to get that code or are you guessing that that is the problem
mostly with O2 sensors a wash and scrub in vinegar will get them working again but the HO2S sensors ( heated sensors ) in front of the cat converters give the most problems from the current in the wires or other circuitry faults
run the fault codes and save money

Some cars can not have a generic converter put on them. Replacing this and the 02 sensors should have fix the code problem. Depending on your car I would check the converter. And if you had it done at a shop take it back. If your car has to have a specific converter and it got a generic one, this would eventually cause the car to stall.

Check your MAF sensor readings. ( MASS AIR FLOW ). You probably need to clean out your throttle body. If it's not reading properly, it could def. kill your car. Hope this helps, James Booth. Please rate this. :))

po420 is catalyst system efficiency below threshold. meaning is--- could be down stream 02 sensors or a bad convertor. try buy replacing down stream sensors, hopefully this may help. ( down stream 02 sensors are between convertor and tailpipe). have a good day !!

PO420--Catalyst System Efficiency Below Threshold (Bank1)
PO430-- " " " " " (Bank2)


O2 Sensor
Coolant Temp Sensor
High Fuel Pressure
Bad Converter
Thermostat
Etc

Owning a 2004 V8 myself--I will say your need to go thru the entire engine, as to how it is running
It may be overheating due to coil misfires or other issues and ruined the
converter

You need to solve all issues before replacing a converter
I would stay far away from a dealer, unless you have a couple thousand to spend

Hello and welcome. the code P0420 is for a bad cat. converter. replacing 02 sensors wont repair this code. the code simply's means that thje cat. converter is not working as designed and will need to be replaced.

The PCM determines the oxygen storage capability of the catalytic converter has degraded below a calibrated threshold.

Huge problems with these cars. The Pre-cat converter is built into the exhaust manifold. Expensive to replace. No Recall on this item. Some say that inferior compounds in the catalyst cause corrosion and early wear of the piston rings. I have seen one of these use 5 qts of oil in 300 miles. Is this car using excess oil?

I can try to help. The chk engine light is generally tripped by the #2 02 sensor. The one after the cat converter. It 's purpose is to monitor the efficiecy of the cat converter. You state there is a power loss problem also. This loss of power is common when the cat converter is restricted and the exhaust can't escape as fast as needed. . You can tell the converter is bad by a sulphur smell in the exhaust, a wheezing at the tail pipe, Or, the exhaust does not immediately stop when theengine is turned off. Good Luck. Have the cat conv checked and replaced . I hope yhis is the help you needed, Ned

The only thing that will set this code is the catalytic converter or the oxygen sensor behind it. Way to tell if it is the sensor is to monitor voltage reading with scan tool and see if it is fluctuating the same as the upstream oxygen sensor. If it is the converter is "free flowing" and will have to be replaced. If the sensor is "flatlined" then it is most likely bad and setting the code.

Catalyst Effeciancy below threshold (Bank 1) is the code meaning. Check or replace the oxygen sensors. Some cars have 2 .One is on the front of the cat. the other is the on the back. Check the back sensor . Other possible causes are leaking or damaged exhaust manifold, bad coolant temp sensor.If toyota did not replace both . replace the one on the back of the Caty.