Updated date:

Emissions Codes P0420 and P0430: Catalytic Converters


Emissionguy is a professsional car mechanic who enjoys the outdoors, dogs, guns, riding, relaxing in the sun, and pool.

"Check Engine Light" Codes P0420 and P0430

Replacing a catalytic converter or "cat" has become a big, expensive drain on a lot of people's budgets these days. I want to share some information about what's happening here with emissions testing, and some ways to get emissions-compliant without shelling out piles of money.

So your car or truck is running well, but that "check engine" light is on, with a code P0420 or P0430, and related "catalyst efficiency below threshold" codes. You like your car, so you take it to be looked at. You choke on the estimate of $1400 or more to replace the converter on your car.

Why Do Converters Go Bad?

Some mechanics will tell you that a converter never goes bad on its own: there is always something else wrong with the car. There is a lot of truth to this.

A converter reduces pollution through chemical reactions that require fuel (from the unburnt gas in your exhaust stream), heat (from your exhaust), and oxygen. If these inputs aren't in balance, the cat doesn't work well.

When a car's engine is skipping, the overheating that results can destroy the catalytic converter. When a car is running too cold from a long warm-up time, for example, let's say if the thermostat is bad, the same thing can happen: good-bye converter.

Another way a converter can die is from poisoning due to the introduction of silicone into the exhaust. When a car engine has a head gasket leak, even a small one that doesn't make the car overheat, the silicone from the coolant can stop the converter from working.

Converters that have failed in these ways can often be brought back to life using a cleaning product called "cataclean." You run the fuel tank low, dump one can of this liquid into your tank, drive for twenty minutes, and then fill up the tank. This actually works about half the time. Then you clear the engine light, drive two days, and go back to emissions for a retest. But I'm not here to hawk overpriced cleaning chemicals.

If you are going to spend the cash to replace a converter, consider replacing the thermostat at the same time, and putting a can of alumaseal in the radiator, so the new cat has a healthy lifespan.

The Role of the Rear Oxygen Sensor

Let's talk about how your car knows the converter is bad in the first place. The rear oxygen sensor (the one behind the converter) monitors how much oxygen is coming out of the converter at any time. A healthy converter will use a lot of oxygen, and so the rear oxygen sensor will see that there is little oxygen coming out of the back of the converter. In this situation, the "check engine" light will not come on to indicate catalytic converter failure, because all is well.

When a converter is bad or failing, the rear oxygen sensor shows a lot of oxygen coming out of the converter, and therefore the light comes on to show that the converter is not healthy.

There is a way to trick the rear oxygen sensor into thinking the converter is okay. Remember how coolant leaks can ruin converters by letting silicone get into them? Well, it turns out silicone also kills oxygen sensors.

Proceed at your own risk because modifications to your car's emissions controls can cause permanent damage to your car. I am not responsible for anything you do to your car. This information is provided only for test purposes on cars not intended for road use.

Here is a huge secret that has not made it into the public domain yet:

If you remove a rear oxygen sensor and pack it with red RTV silicone, that will destroy the oxygen sensor in a way that will trick your car's computer into thinking the catalyst is okay. The sensor will not work well enough to report that extra oxygen is coming out of the converter, but it will still generate enough voltage to pass the car's oxygen sensor activity test. Never do this to a front oxygen sensor because it will mess up the fuel system.

Well, there it is, one of the best-kept secrets in the smog-loving world. This works nearly 100% of the time. You will never find a mechanic that will do this for you, so you had best be prepared to remove that sensor yourself and put it back in.

Certain newer cars will not tolerate having the rear oxygen sensor dummied up, so you will have to get a new sensor after passing, or the car will start to underfuel and you will get a code for "fuel trim rich." Some Chrysler and Mercedes cars actually use info from the rear oxygen sensor to adjust the fuel, so you need a working rear sensor.

If you want to restore the car to normal working condition you will need to get a new oxygen sensor, because any sensor that has been siliconed will never work properly again.

Which Sensor Bank Is Which?

Bank One is always the side of the engine with the number one cylinder. Check the firing order to determine for sure which side has the sensor you are looking for, Bank One or Bank Two.

Another Trick for Two-Converter Cars

Another mechanic sent this other converter trick to me, and I tested it today and it did work.

Let's say you have a car with two converters and four sensors. If only one of the converters has failed, you can sometimes connect both rear sensor signal wires together and the average of the two makes both look good in the eyes of the PCM (Powertrain Control Module).

Today I had a 2001 Sentra with four sensors even though it was just a four-cylinder. The car had a P0430 error message for the Bank Two converter only. The rear oxygen sensor wire connectors were right next to each other under the hood, so I just jumpered the two signal wires together and ran the drive cycle. Problem gone.

I hope this article has helped you learn about catalytic converter failures. Best of luck to you.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Comments Welcome!

Max (author) from USA on October 19, 2018:

@dpfast Sensors never cause a P0420. Exhaust leaks and bad cats. The mode6 data could be because that monitor has not run yet in that drive cycle.

DPFAST on March 28, 2018:

Great suggestion on the Red RTV. I have 02 RAV4 with 239K and already went through 4 CATs since new. My last replacement a year ago along with all 4 new sensors by Denso (2 A\F and 2 O2) and now again P0420 and P0430. No other codes and the car runs great. As a Toyota Specialist I was wondering if you could maybe help me understand something? When scanning the onboard modules, the $01 module is faulty but no codes. Since they are Toyota's proprietary info I am not sure what that means? I think its related to the A\F sensors as it says :

TID: $01 CID:$01 Rich to Lean sensor threshold voltage (constant) Max: 128 Current 154

Same thing for the $02 rich to lean is high. I am thinking that maybe my new Denso A\F sensors are at fault which in the end causes the P0420 codes? Could you please clarify this for me? I would really appreciate it. If I can't figure this thing out I will try your trick with the RTV as I have to take the smog test soon.


hardlymoving from Memphis, TN on August 16, 2011:

I had a converter go bad on my 94 Ford Explorer. All I did was buy myself a universal 3 way converter for under $100, cut out the old converter leaving enough pipe to fit inside the new converter then weld in place. And you got people getting prices quotes of $1,400 for a new converter?

Max (author) from USA on August 10, 2010:

Some vehicles are prone to the catalyst collapsing causing the exhaust to become clogged but this condition does not come and go, it gets gradually worse and is often associated with a hissing noise.

Instant-Immersion from Saint Louis, MO on August 10, 2010:

Can the catalytic converter deteriorate to the point to where it affects the running of the engine? I've been doing a lot of basic tune up stuff, but I'm wondering if my catalytic converter may be the problem. My van has 190,000 miles on it, and it runs rough sometimes when first started, but it usually smooths out later.

Related Articles