Just replaced O2 sensor a few weeks ago and check engine light has returned for yet a second time. Code is now reading P4020 Catalyst efficency below threshold.

Question edited for clarity.
Question moved to model category.

You say you 'changed the spark pug' (singular), did you only change one? When one fails, the rest are going to follow.

Your O2 Sensor and O2 Heater are probably the same sensor that is failing. When they fail and are running lean, they dump fuel and that soots up the Catalytic Converter, so there is your other code. Replace the sensor that is causing the issue and run some Extreme Clean Catalytic Converter Cleaner in the fuel. You might need two treatments.

You need a good auto electrician

https://www.google.com/search?q=auto+electrician+near+me

P0430 Code: Catalyst System Efficiency Below Threshold ...

CarParts.com https://www.carparts.com > blog > p0430-code-catalys...
Oct 29, 2021 - P0430 is a diagnostic trouble code (DTC) for "Catalyst System Efficiency Below Threshold (Bank 2)." The code warns about a problem with the ...

This is a good gadget if you want to take control of your own codes. It talks to an App on your Smart Phone. I have one.

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Got it scanned and it's P0420: Catalyst efficiency below thresholdbank 1. They said it could be a bad catalytic converter or oxygen sensor. I borrowed a scanner and tried to measure the oxygen sensor readings over time.Sep 8, 2011


civic - Check Engine code P0420 Catalyst efficiency below ...

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P0420 Error Code: Catalyst System Efficiency Below ...

www.obd-codes.com/p0420
P0420 OBD-II Trouble Code. Technical Description. Catalyst System Efficiency Below Threshold (Bank 1). What does that mean? This diagnostic trouble code ...

P0420 Code Fix How Its Done Catalyst Efficiency Below ...

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Jun 20, 2014 - Uploaded by ACE AUTOS UK
Here is a tutorial on P0420 code fix catalyst efficiency below threshold How To Diagnose a p0420 code ...

There is no such code for any car. this 1999 cougar with the 2.5 engine has a code and i think you may have mixed the numbers the only code thats close is the po420.

Code po 420 is the cataylic converter efficency, this means the converter is not cleaning the exhaust to specs anymore and may be bad.

I have found that i lot of times a bad o2 sensor is the cause of this or both o2 sensors as in this car,i have found that its good thing when a converter is replaced that the o2 sensors should always be replaced. this will cost more but is the correct way to do the job. what i do is replace the o2 sensors first and then reset the light and then if light doesnt come back problem is fixxed.
there are mant people out there that say a good tech can see if the sensors are bad, ture and not,you can test a sensor by watching the reading and also changing them to see how fast a sensor can react and theres more to it but i wont bore you.


MY point is , i have replaced bad o2 sensors and then the converter is bad, now the customer is not happy as the quote just went up alot. then i replace a bad converter and then find an o2 that worked a week ago is now slowed down,
This is to keep in mind if its a converter and the o2 sensors are old replace them, they may be working now but lets say they slow down but not enough to turn engine light on which i see alot, then a year later you wonder why you must buy another converter.

Good day.

is that the code you get from comp is PO420 eng light if so worst news either you have an exh leak between o2 sensor 1 and o2 sensor 2 or the front o2 sensor is lazy or the front catyletic converter is not working

basicaly code is only triggerd when engine is hot and the front o2 sensor and the rear o2 sensor read the same voltage when conv is working the front o2 sensor will fluctuate rapidly from 0v to 1v and rear o2 sensor will remain steady at 0.5v

try to explain best i can plz vote comment or reply if you need more help

you may have a bad converter, its not cleaning the exhaust, but on the other hand a bad rear o2 sensor can also cause this problem,there is a wat to test but you would need a scan tool or scope,replace the other sensor, if light returns then replace the converter, no money lost, if you were to replace the converter the o2 sensors should also be replaced. you have 165000 miles it would be crazy not to replace sensor and install new cat, so basically your just doing it in 2 steps and may get lucky and not need a cat. good luck.

There is an oxygen sensor (o2 sensor) in front of the catalytic converter, and another one behind it. The computer looks at these two readings and if they are close to the same, it determines that the converter is not working efficently. It could be bad o2 sensors, or a bad converter. You will have to scan the computer and see what the readings for these two o2 sensors are. If they are both close to zero, then replace the sensors. If they are both reading voltage from the sensors, the converter is bad.

it means the catalyst is worn out, the efficiency is monitored by sensors, when the Cat falls below a certain reading or "threshold" the code is generating telling u that u need a new catalytic converter.

Hi,first thing your cambelt and tensioner should have been replaced 13k miles ago,it would pay you to replace the water pump at the same time as they are known for impellor failure = cooling system fault=P2181 code,hope this helps.
Darren

Check and see what the codes are this time, no need to replace something you don't have to.

Your number 1 O2 sensor is bad replace it. this is the O2 sensor attached to your headers. Could be faulty sensor or the air/fuel ratio is offset. But replace the sensor its not running effeciently.