Check the oxygen reading of bank 1 sensor 2. Also look for any leak on the exhaust system. How many miles you got on the car?

If you don't need to pass the emission test, why do you want to fix it? 

Btw, not all the cat issues are related to the O2 sensor. How many miles you got on the car and did you check the live data for the O2 sensor?

It could be an O2 sensor.

It could be a bad cat.

Check the O2 sensor data.

It most likely is a bad cat, just based on prior experience.

I’ve had friends that straight pipe their cars.  But that is illegal, even in a state with no inspections.

https://www.autocodes.com/p0420_honda.html

Have a look at this.

https://youtu.be/0FDXAmj5-hY

What, exactly, is the spacer supposed to accomplish?

2007 GMC Yukon, 88K mi. New plugs, wires, alt, and battery. Changed O2 sensors up and down stream on passenger side. Code went away then came back after 500 mi. Have done seafoam spray down intake and cleaned maf sensor, stays clears for 40-60 mi. then comes right back. Suggestions?

Car Wizard

https://www.youtube.com/watch?v=SwWcPPjoRwY

I have a 2005 Subaru Outback non-turbo 2.5 engine with an automatic. When I bought this car it threw the code P0420 about a week after I picked it up. The car place still had a warranty on the catalytic converters so they got me a new set that did not fix it! Replaced upstream and downstream sensors and that did not last long abd the code showed up again. last ditch desperation move was the mass airflow sensor! What should I do?? I was told by Subaru you have to buy our special cats and pipe setup for $1800!!?? I dont think so! HELP!!!

Did you check to make sure the wiring was ok?

Confirming what @HillBilly said. 

I have the same issues with my EJ251 with a universal cat. Mine will sometimes go away for a couple hundred miles, then come back. Subarus are picky about cats and the genuine ones cost so much more because of their higher precious metal content. If you do have a scan tool, your LTFT should ideally be close to zero, but even older Subaru motors shouldn't be tripping P0420 with LTFT inside of ±5%.

If you live in a place that doesn't have emissions inspection, your state inspection may allow for a car as old as yours to pass with one emission system (say, the catalytic converter) failed. As long as your car doesn't have any other emissions component failures (say, EVAP or Spark), you could keep on driving it with the code for many more miles.

Last week I had a P0330 code for which I got my knock sensor and harness replaced. After I got my car back I see the check engine light come on with P0420. 

Now I am not sure what’s wrong and was it caused by the repairs that were done. 

Can I keep driving with this code or do I need some immediate repairs? 

PS: I haven’t noticed any drivability issues.

This code means that the catalytic converter is not operating as efficient as it is supposed to. So more than likely your cat is bad. You can drive like this forever but you won't be able to pass emissions or inspection if you have to worry about that.

@batcave
The function of the catalytic converter is to break down harmful pollutants that are created by your Toyota during the combustion cycle and convert them into harmless gases. Code P0420 indicates that the catalytic converter is not functioning efficiently, therefore increasing the output of harmful pollutants by your Toyota .
P0420 Causes
-Faulty oxygen sensor
-Worn and internally failing catalytic converter
-Leak in exhaust system .
Generally this is not gonna harm your engine except when it's arrive to a clogged catalytic , and if it's caused by a bad o2 sensor it can lead to a bad gas mileage . Any way it doesn't have anything to do with the knock sensor repair

I don’t think it has anything to do with the gas cap, that has its own code in my experience. Fuel evap system malfunction or something like that. 

Hey Scotty i got a 2000 Toyota celica 6 speed manual with 200,000 code po420 the car got a high idle when warmed up. No vacumn leaks could it be my maf sensor cause i checked it and the housing were the wire be it looked damaged. The iac valve is bra...
{black}:excitement: {black}:grinning:  

Hello 2000 Toyota celica gts 200,000 miles code p0420 high idle when warm 1300 rpms and gas pedal sticking could it be a throttle body problem???
{black}:idontknow:

I have a 2008 Honda Pilot with an error code P0420 "Catalyst systems efficiency below threshold Bank 1".  The car has 125k miles and I bought the car used at 98k miles.  I have changed the spark plugs and the check engine light went out and then came back after about 150 miles.  So, far I have not seen any performance issues.  The prescribed fix is to change the catalytic converter.  I would like to avoid doing this.  No other error code are on the car.  Can I let the car run with this check engine light on?  Do I risk damaging the car?

Bank 1 rear cat is not working efficiently, which means you have may have a bad cat. 

How many miles on the car and does it burn oil? 

CEL will come back on soon or later. 

Hi Scotty,

I have this vehicle with a PO420 and pretty sure the catalytic converter is bad.  it is sluggish, low gas mileage, also sometimes burns oil.  I may find it 2 quarts low in a short period of time or will be fine for the same period.  Doesn't seem to be linked to usage.  I've replace the PCV too.   Intermittent and weird.  I'm somewhat inclined to do replace cat myself but it is just out of my comfort level.  Also, do you have a ballpark of what I should expect to pay?  It's a cool looking car and my daughter loves it but am also wondering if I should pull the plug on this because not sure it's worth it.

2004 Nissan Xterra - 2 wheel drive, automatic

225,000 miles

3.3L v6 non-supercharged

thanks!

Arnold

"Doesn't seem to be linked to usage."

Does daughter let boyfriend drive car?  Ours did.  I could tell when they met, when he was doing driving.  Seems she used sense driving while he had no respect for her car and dogged it.  I followed and saw him driving abusively.  I changed how she was allowed to use car and who drove it.  More for her safety than for fuel costs.

A car driven by the same person will wear the evenly.  Odd wear can mean bad car or different drivers

Not saying the code is because of abuse but there could be two reasons for what you see.

Muffler shop.

Hello @scottykilmer

I own a Acura TL 2006 with 178000 miles on it. I too have a PO420 Error Code on my car as you have mentioned in the Subaru WRX Wrecked Title ( https://youtu.be/rBjJp3K20FQ). Here are the sensor readings on my car.

Fuel Trim(long) B1 - 4.7%
Fuel Trim(long) B2 - 7.8%
Fuel Trim (short) B1 - 10.9%
Fuel Trim (short) B2 - 10.9%
Oxygen Sensor 2 Voltage - 0.61V
Oxygen Sensor 6 Voltage - 0.5V

Can you please help me in resolving this Issue

Thanks

Hi. I have a 2003 Honda CRV, automatic, with around 115,000 miles. I've had a code P0420 for a while now. I've tried replacing the spark plugs and that didn't work. I've tried the lacquer thinner cleaner trick and that also did not work. There's no smell, rattling, lack of performance, or even any rust on the cat. I'm thinking about getting an aftermarket one from RockAuto, a Walker CalCat (WALKER 82896) to replace it if it's bad. Has anyone tried Walker or another brand of catalytic converter aftermarket? Looking at $350 for that vs. $2000 from Honda OEM. Also, any other tricks to get my old one working again? Thanks!

Hey Scotty I have a 2000 Toyota solara with 103,143 miles and I have a check engine light on for my catalytic converters I have a p0420 code the car needs to be inspected by March do you think I should fix it or is it time to sell my poor little baby thank you and stay safe 

Hi my sister has a 09 civic hybrid 166k miles everything works fine except she can’t pass smog she throwing the po420 code catalyst system below threshold bank 1 what could it be a o2 sensor? She’s has monitors reset and this is still showing 

on another topic what year Corolla 2014 or newer has the best drivetrain , were looking into a newer/used car for her 

How's it going Scotty! I made sure I watch all of the vids you posted referencing Catalytic Converters...My CHECK ENGINE LIGHT just came on today; I used my Scan Tool to Read the Code found...it showed P0402 Catalyst System Low Efficiency Threshold Bank 1. Ok. now my cars' operation doesn't feel abnormal but still I'm wondering should I simply clean the current catalytic converter or purchase an aftermarket one???

Hello,

I have replaced O2 sensors (heater core failures), adjusted valve gaps, sparkplugs, coils and air filter. Now, I am getting P0420 B1 codes. I tried 1 Gallon of Lacquer Thinner. This does not seem to change the code. 4 years ago, Honda claimed that the B1 Cat was nearing end of useful life. Has it? 

The engine has 300,000 miles on it. Runs well. Autel says the B1 and B2 are Ok. My downstream Denso O2 sensors both are about 1.07V. I think this is what is tripping the codes. I am not given a percentage. I am ready to replace both. What should be the plan? 

I have a 2012 GMC terrain with 192k miles on it. It's throwing the po420 code, I used later thinner and though the engine never ran poorly it didn't clear the code. Would doing another round of lacquer thinner/ driving on the highway be a useless or bad action?