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P0420, came on after i change the cat converter, should i change the o2sensors

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1st off, P0420 is not an O2 sensor code, it is a catalyst efficiency monitor code, this means when the measured limits allowed exceed a certain limit for the clean up of the exhaust gas stream the Powertrain control module (main computer control) will turn on the check engine light and store code P0420 to signal the Catalyst is worn out, this sensor looks like an O2 and is located after the main catalytic converter in the exhaust system, many factors can trigger the light to come back on, a defective sensor is possible and replacing the one sensor after the cat may cure the problem other factors that affect the cat performance are using an aftermarket catalytic converter and any type of exhaust leaks ahead of the cat converter, loose head pipes or leaking exhaust manifolds or rusted out pipes, anything that leaks ahead of the catalytic converter, so do this, replace the cat efficiency sensor aft of main cat and check for any type of exhaust leak before the main cat converter, if the code comes back an u did use a universal replacement that didn't come from GM that is the problem, and that is the common issue, aftermarket cats in many cases just don't clean up the exhaust enough to satisfy the parameters of the powertrain control module that constandly monitors the output of the cat converter, you can also read more about the rather confusing subject at the web link below, it is solely dedicated to the subject of OBD2 code P0420. the info i have just given u was learned at GM factory schools while i was a GM dealer tech, my speciality is electronic engine control .

Posted on Oct 18, 2010


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Where is the O2sensor bank 1 at with a code of P0420. AND do I need to replace both sensors upstream and down. Or just the one that is showing up on the obd scanner.

Bank one is front side of cat. It should also tell u if it's cat 1 or cat 2. Depending on make an modle an yr. Catalyst 1 bank 2 is left cat back sensor. Catalyst 2 bank 2 is right cat back sensor. U do not have to replace upstream also. My o2 sensor Catalyst 2 bank 2 went bad. Had engine code for burning rich bank 2. Replaced tha down stream sensor an fixed problem runs better an better gas. This was in 97 gmc suburban. Idk what vehicle ur askin about. An I'm not to familiar with much about cars jus trucks an suvs.

Jan 02, 2017 | Cars & Trucks

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How to fix p0404 and p0420

code p0404 refers to exhaust gas recirculation (EGR) valve position--range/performance problem ===causes --hose leakage/blockage--wiring ---EGR valve/solenoid
code p0420 refers to cat converter system bank 1-efficiency below threshold===causes --cat converter---wiring---HO2S 2

May 31, 2014 | 2007 Dodge Grand Caravan SXT Minivan

1 Answer

2003 chevy trailblazer code p0420

Is your engine temperature at 210 degrees? Need a scan tool for this, the guage isn't reliable. If not, cheapest thing to do 1st would be change your thermostat if it's not at 210. If that doesn't work, have your catylitic converter back pressure tested. You could try some Cat Clean added to your gas too as cats are expensive.

Feb 28, 2014 | 2004 Chevrolet TrailBlazer

1 Answer

Which o2 sensor to change for codes p0420 in 2005 mazda 3 where the catalyst is missing

No cat this must be a middle east model then ,they have no cats or O2 sensors

P0420 Catalytic Converter System -Efficiency Below Threshold (Bank 1)

Which makes a mockery of europeon anti poloution laws let alone californian controls .These middle east countries should be forced to introduce emmision control laws .

Nov 02, 2013 | Mazda Cars & Trucks

1 Answer

I was driving my car and the cruise control light started to blink and my check engine light came on it is a 2005 subaru legacy i took it to auto zone the code was p0420 they said it could be a o2sensor or...

if you get a rotten egg smell from the exhaust it could be your cataylic converter. lots of times the motor will say its an 02 sensor issue when the cataylic convertor has had enough.

Oct 03, 2013 | 2005 Subaru Legacy

1 Answer


P0420 indicates that the catalytic converter efficiency is low. The usual fix is to replace the catalytic converter. If the exhaust is not leaking and there are no codes for the rear O2 sensor you'll more than likely need to replace the cat to get the code to go away.

O.E. cats as well as direct fit aftermarket cats from reputable manufactures are the best route. After market universal fit cats are not worth the headache and usually cause more problems.

Nov 01, 2010 | 2000 Volkswagen Beetle

2 Answers

Engine code p0420

The P0420 code is set when the oxygen sensor after the catalytic converter gives a reading the same or very close to the reading of the oxygen sensor(s) before the catalytic converter.

There are usually three possible causes...
The front oxygen sensor (before the cat) is giving an incorrect reading.
The rear oxygen sensor (after the cat) is giving an incorrect reading.
The catalytic converter is not working properly.

Since you recently replaced the cat, the logic points to the sensors, but it could be the cat again. Aftermarket converters are usually smaller than OEM and don't last as long.

For a more thorough description of the code and troubleshooting visit

Our Subaru encountered this issue several years ago under warranty. The first time the dealer replaced the front oxygen sensor. This was the cheapest repair and since it was an intermittent problem I am sure they hoped to delay the issue until the warranty was up. The second time they replaced the catalytic converter and we haven't had any more issues.

Jan 08, 2010 | 2000 Honda Civic

3 Answers

Receiving engine code p0420 on my 2005 altima 2.5 s

A p0420 code indicates a Catalytic System efficiency failure, Bank 1.
Basically, the computer is seeing reduced efficiency of the catalytic converter or one of it's components (such as an 02 sensor)

There may be a few root causes, such as a faulty oxygen sensor, faulty coolant temperature sensor, faulty converter, Large exhaust leak, or retarded (slow) timing.

Nov 30, 2009 | 2005 Nissan Altima

1 Answer

On my 2003 do i have change both cat?

Did you reset the light after changing cats?

Jul 05, 2009 | 2002 Toyota Camry

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