p0420 code – meaning, causes, symptoms (& how to fix)

if your car’s check engine light comes on and you see the code p0420 on your code reader, do not panic. this article will help you understand what this error code means, what could be causing it and how to fix it.

we know from experience that p0420 can be quite tricky to get rid of without the right knowledge. one of the main reasons for this is that if you replace a part and reset the codes, it can take a very long time before it reappears.

in this blog post, we’ll answer all these questions and more. keep reading to learn everything you need to know about the p0420 code.

code definition

p0420 – catalyst system efficiency below threshold (bank 1)

what does the p0420 code mean?

code p0420 stands for “catalyst system efficiency below threshold (bank 1)”. it simply indicates that the ecm has detected underperformance of the catalyst efficiency on bank 1. it can either be caused by a faulty catalyst or a false alarm caused by a faulty o2 sensor.

the engine control module (ecm) uses two o2 sensors, one in the front and one in the back of the catalytic converter to measure efficiency. if the catalyst system efficiency is low, the p0420 code will be triggered. in most cases, a p0420 trouble code is caused by a bad catalytic converter. an incorrect air-fuel mixture can also temporarily or permanently reduce the efficiency of the catalyst, resulting in a po420 code.

p0420 symptoms

the most common symptom of code p0420 is a check engine light on the dashboard. in some rare cases, you may also notice issues like misfires, higher fuel consumption, or a bad odor smell coming from the exhaust pipe.

however, in most cases, you will most likely not notice any symptoms except the check engine light with the p0420 code stored in the engine control unit or powertrain control module (pcm).

how serious is the p0420 code?

low – the p0420 code will in most cases not create any further problems with your car’s engine.

the only thing that could potentially happen is that the catalytic converter is so damaged that the catalytic converter’s internal parts come loose and block the exhaust flow, which is pretty unlikely to happen though.

but, the p0420 trouble code could make your car’s emissions bad, which is bad for the environment, so you should fix it as soon as possible.

p0420 code causes

the most common cause of the p0420 code is a faulty catalytic converter. it can also be caused by a faulty oxygen sensor, exhaust system leak, intake leak, or incorrect placement of the catalytic converter.

it may be a defective bad catalyst, but i have also seen many cases when the catalyst is new, but it is not an original oem part. some cheaper catalysts may not be efficient enough and it could result in that you may need to buy a brand new catalyst from the manufacturer.

i have also seen non-oem catalytic converters that have been installed too far away from the engine on the exhaust pipe towards the last muffler. because of this, the catalytic converter will not get hot enough and it will trigger the p0420 trouble code.

if the car’s air-fuel mixture is not correct due to a faulty injector, high fuel pressure, faulty fuel system, or a damaged sensor, it may cause the catalytic converter to not function properly. here are all possible causes of p0420:

• damaged catalytic converter (most common)
• oil contamination in the catalytic converter
• incorrect placement of the catalytic converter
• damaged upstream or downstream oxygen sensor / faulty wirings
• exhaust manifold or exhaust system leak
• wrong type of fuel (leaded fuel instead of unleaded)
• rich/lean mixture (damaging the catalytic converter)
• misfires (damaging the catalytic converter)
• a faulty engine control module (rare)

what repairs can fix the p0420 code?

you should first check the function of the catalytic converter with the methods further down in the article before replacing any parts. you will most likely waste your money by just replacing parts, so it is much better to make a proper diagnosis. however, here are some things that could fix the error code p0420:

• cleaning the catalytic converter
• replace catalytic converter
• replace front or rear oxygen sensor
• repair damaged sensor wiring
• fix oil burn
• fix misfires
• fix lean or rich fuel mixture
• replace engine control unit (rare)

common p0420 diagnosis mistakes

the most common mistake is to replace the oxygen sensors without doing a proper diagnosis. the cause of this engine code is in most times the catalytic converter – which may get damaged by other problems with your car’s engine like misfires or a rich or lean air-fuel ratio.

check for any other trouble codes with your diagnostic scanner. if you find codes like p0300, p0301, p0302, p0303, p0304, p0305, p0306, p0307, and p0308, you have a misfiring engine and the misfires may cause the p0420 error code. in this case, you should fix the misfires first, clear the codes and see if the p0420 code comes back.

if you find dtc codes like p0171, p0172, p0174, or p0175, it means that your engine is running rich or lean. this could be due to a faulty fuel injector, high or low fuel pressure, maf sensor, engine coolant temperature sensor, or almost any faulty engine sensor. the air-fuel mixture needs to get addressed and then see if the p0420 code comes back.

in some cases, if there are no other issues with your engine, you can try to use a catalytic converter cleaner before replacing the catalyst. there are many different additives on the market, so we recommend choosing one of the best catalytic cleaners from our list.

common p0420 code causes by car model

the p0420 trouble code is more common in some car models than others. these car models are known to have a problem with this diagnostic trouble code. here is a list of the most common causes by car brands.

remember that these are only 2022世界杯榜 guidelines, and you should make a diagnosis before replacing any parts.

1. chevrolet

if you experience a p0420 code in your chevy it is often due to an exhaust leak before the catalyst. it can also be caused by a faulty downstream oxygen sensor (rear) or a faulty catalytic converter.

chevy engine’s are known to leak around the exhaust manifold so you should definitely take a look there if you can find some strange blowing sounds when your engine is running.

2. toyota & nissan

the most common cause for p0420 in toyota and nissan cars is a bad catalytic converter. you may also ensure your engine is not burning motor oil.

check for vacuum leaks and exhaust leaks first. then check to see if you notice any blue smoke coming from the exhaust pipe. if so, it’s a sign that you might have an issue with the crankcase ventilation or even something as bad as a damaged turbocharger or bad piston rings.

if you do not notice any blue smoke at any rpm, it is most likely your catalytic converter is worn out. you may find similar issues on nissan cars.

2. ford

ford engines often have vacuum leaks or a broken solenoid for the egr or other components, which causes a faulty air-fuel mixture and then causes the trouble code.

check the code memory with a diagnostic scanner to see if you can find any trouble codes about the air-fuel mixture. if everything looks fine, check for exhaust leaks.

diagnose and replace the catalytic converter if you can’t find any trouble codes or other problems with the air-fuel mixture.

3. subaru & honda

if you find a p0420 code in your subaru or honda, it’s most likely the catalytic converter to blame. it can also be caused by an exhaust leak or a bad oxygen sensor.

check for vacuum leaks or other fuel mixture-related engine codes. check for any exhaust leaks before the catalytic converter. however, the most common problem with subaru engines is the catalytic converter due to the fact that they seem to wear down fast on these boxer engines.

4. volkswagen (vw) / skoda / seat / audi a4 1.8t / v6 2.4

these vag cars have some known problems causing the p0420 trouble code. check the check valves’ function under the intake and make sure the crankcase ventilation is free from dirt, causing the engine to burn oil, which clogs the catalytic converter.

check for exhaust leaks around any flex pipes on the exhaust pipe (common cause) and check for any engine codes related to the oxygen sensors. if no problems were found, replace the catalytic converter. it’s a widespread problem on both the 1.8t and the v6 petrol engines.

recommended tools for diagnosis

• diagnostic obd scan tool
• multimeter
• basic hand tools
• laser thermometer
• auto repair manual
• electrical contact cleaner

how to diagnose code p0420

the p0420 code is mostly caused by a faulty catalytic converter as mentioned before. anyways, you should always diagnose it properly with the methods down below before replacing any parts.

1. connect an obd2 scanner and look for related trouble codes. repair any related trouble codes related to the ignition or fuel first.
2. check the live data to see the front and read the o2 sensor voltage signals. the car engine should be scorching – and the front sensor should fluctuate between 0-1 volts, and the rear should be steady at 0.7 – 0.9 volt. if you get similar readings, there is a chance that the catalytic converter is defective.
3. heat up the engine and check the temperature of the front of the catalytic converter and then the rear with a laser thermometer. if the engine is hot and there is no difference in temperature before and after the catalytic converter – your catalytic converter may not work as intented.
4. if the catalyst is installed available, it may be worthwhile to remove the pipe from one end of it and check inside the catalyst for visual damage.
5. if everything points at a faulty catalytic converter – replace it. if you can’t find any problem with the temperature, voltage, or a visual inspection, you should try to repair other related trouble codes and then clear the codes and try again.
6. if you still can’t find any problems. make sure it is a genuine oem catalytic converter, and that it’s installed in the original place. if everything seems fine – replace the catalytic converter.

video for more advanced 2022年世界杯淘汰赛 :

catalytic converter damaged causes

the catalytic converter is there for reducing the three main causes of pollution produced through gasoline combustion – nitrogen oxide(nox), carbon monoxide and hydrocarbons. it does this by converting these harmful substances into less noxious ones that are easy on our health plus they contribute to clean air quality!

there are a few things that are known for damaging the catalytic converter, which can cause the trouble code; here are the most common:

• misfires
• oil consumption
• exhaust leak
• intake leak
• rich mixture
• lean mixture
• bad ecm/pcm

estimated repair cost

there is no fixed cost to fix the p0420 code, due to the number of different causes. however, it is often caused by a faulty catalytic converter, which usually costs between 500$ to 1500$.

the estimated cost of repairing the code is the following. the prices are including parts and labor work at a workshop. the costs do not include diagnosis costs.

• catalytic converter replacement – 500$ to 1500$
• front oxygen sensor replacement – 150$ to 300$
• rear oxygen sensor replacement – 150$ to 300$

can i remove any parts to get rid of the p0420 code?

you cannot just remove any parts to fix this trouble code, because it will most likely give you another trouble code or other issues. but, you could reprogram the engine control unit to take away the catalytic converter monitoring. still, it’s not recommended because, in most countries and states, it will be illegal.

if you want to reprogram the function, you can also remove the catalytic converter completely. remember that if you remove the catalytic converter, you will most likely not pass any emission tests and you may experience legal issues.

there is also another way to trick the engine control module by replacing the rear oxygen sensor inside of a small pipe. this method can work if you desperately want to get rid of the p0420 code, but it’s not a recommended method, and it may get you into trouble.

how to clear code p0420?

to clear the p0420 code, you must use an obd-ii scanner. keep in mind that just clearing your p0420 code will probably only solve your problem temporarily. to fix it permanently, you need to fix the underlying issue.

frequently asked questions

related codes:

p0430 – catalyst system efficiency below threshold (bank 2)