In my 20 years experience as a master technician, it is always the Catalytic Converter that needs replacing when P0420 Code sets the CEL (Check Engine Light) on.
Can you explain in simple terms what P0420 Code means?
Let me put it in simple terms, in order to keep pollution down, car manufacturers have to install extra things in your car.
- The Catalytic Converter is one of those things. Catalytic Converters help accelerate the burning off of any excess gas fumes, so that they do not go out the exhaust as pollution.
- In order to make sure the Catalytic Converter is working properly, car manufactures install a thing called an Oxygen Sensor.
- They install an Oxygen sensor before the Catalytic Converter (Upstream O2 sensor) and one after the Catalytic Converter (downstream O2 sensor). The Oxygen Sensor before the catalytic converter takes a measurement of the amount of Oxygen and sends it to the ECM (Electronic Control Module) or simply known as the car computer, the Oxygen Sensor after the Catalytic Converter takes a measurement and also sends it to the computer.
- The computer compares the reading from both sensors and from that information, it can determine if the Catalytic Converter is working at full efficiency.
- If it is not, then it sets P0420 Code, Catalyst System Efficiency Below Threshold (Bank 1), bank 1 is the side of the engine that has cylinder number 1 if it is a V6, V8 or higher. On straight 4-cylinder engines, it does not matter, because you only have one bank of cylinders.
Does P0420 Code mean a bad Catalytic Converter
In my 20 years experience as a Master Technician, it almost always turns out to be the Catalytic Converter. But there are exceptions. Here are other things that can cause a P0420 Code:
- Bad O2 sensor (Oxygen Sensors)
- Exhaust leak
- Bad ECM (Car Computer)
- Bad Spark Plug
- Bad Spark Plug wire
- Engine misfire
- Leaky Injector
- Problem with the fuel pressure
- Using the wrong fuel or additive that damages the Catalytic Converter
- Contaminants from a failing head gasket
- Oil leaks that coat the inside of the Catalytic Converter
- After market or cheap Catalytic Converters that do not meet OEM (Original Equipment Manufacturer) specifications.
- ECU (Engine Control Unit) or ECM (Engine Control Module) that is lacking a firmware upgrade.
- Using a regular Catalytic Converter on a vehicle made to the stricter standards of the State of California.
- Loose or damaged heat shield causing false sensor reading due to temperature fluctuations.
- A failing EGR (Exhaust Gas Recirculation) valve.
- Bad wiring or connector to any of the sensors, for example, the Oxygen sensor.
- Using high-mileage oil that burns and sends deposits into the exhaust stream, contaminating the converter.
- Vacuum leaks, affecting fuel trims and causing a lean condition that disrupts how the cat works.
- A malfunctioning thermostat causing the engine to run too cold and making the catalytic converter inefficient.
- Using silicone-based sealants near the exhaust system, which can coat the sensors or cat and disrupt function.
- Incorrect engine coolant temperature sensor readings, causing improper fuel delivery and exhaust gas content.
- A failing PCV valve, letting excess oil vapor or crankcase gases into the intake and through the exhaust.
- Using low-quality or counterfeit oxygen sensors that give inaccurate voltage signals to the ECU.
- Engine running with low compression in one or more cylinders, causing incomplete combustion and extra emissions.
- An incorrectly installed or damaged O2 sensor spacer or defouler giving delayed or distorted readings.
- An aging vehicle battery or weak alternator causing low voltage and inconsistent sensor performance.
- Using engine oil with high phosphorus or zinc content that slowly damages the catalytic converter.
Can I still drive a car with a P0420 Code?
If your car is driving normal with no other symptoms like rough idling, misfiring, running hot, or weird smells, it usually means it is safe to drive. In summary, it is safe to drive, but it will be polluting way more than it is supposed to. And in places where you need to get an emissions test for your car every year, your car will fail that smog or emissions test. Also, keep an eye on the Check Engine Light, if the Check Engine Light starts flashing, that means something more serious is wrong with your car. You can use your car for short trips, just don’t forget about it. Do get it looked at, as soon as you can.
Is P0420 expensive to fix?
It depends what is causing the P0420 Code. If it is a simple Oxygen sensor, it can run a couple of hundred dollars with labor and all. But if it is the actual Catalytic Converter or Converters, because some cars have as many as 4, then your repair cost can be several thousand dollars. And to add to that, you might also have to fix what damaged the Catalytic Converter in the first place. Yeah, it can get expensive.
Do Catalytic Converter cleaners work?
Absolutely yes. If your Catalytic Converter is just marginally dirty, a Catalytic Converter cleaner can work. From my experience on 100s of cars, the sooner you clean the Catalytic Converter after the CEL (Check Engine Light) turns on, the better chances of fixing it.
What do you recommend, to clean the Catalytic Converter?
There are many Catalytic Converter cleaners on the market, some are great others don’t work. I personally use and recommend Cataclean. It is sold by Holley, yeah, the legendary carburetor maker. Check out their page on Amazon here P0420Code.com/CatacleanAmazonPage
Should a P0420 Code be fixed immediately?
Any code that turns on the CEL (Check Engine Light), in theory, should be fixed as soon as possible. Now, in practice, unless it is overheating or idling really rough, you can delay the fix until you reach a shop. Driving the car without fixing it might damage other components. So, if you need to, go ahead and drive the car, just don’t ignore the problem completely.
Can worn spark plugs cause P0420 Code?
Absolutely yes. Worn spark plugs will cause the gas:air mixture to not burn completely. This simply means, more unburnt fuel will go out the exhaust. This can cause the Oxygen sensors to give erratic readings, and that will set the P0420 Code.
How to unclog a Catalytic Converter?
You cannot unclog a clogged Catalytic Converter. You can only clean a marginally dirty Catalytic Converter. If when you drive your car, you hear a loud hissing noise coming from the exhaust system, that is a clear indicator that the Catalytic Converter is clogged. At that point the only solution is to buy a new Catalytic Converter.
How much does it cost to replace a Catalytic Converter?
Catalytic Converters start as low as $70 from a non-dealer resellers. Places like Autozone, O’Reilly, and Napa carry quality Catalytic Converter but they start around $100. If you want original factory Catalytic Converters, they could run in the $1000s.
Can I drive with a bad Catalytic Converter?
If your car’s running fine, and you’re not seeing stuff like rough idling, misfiring, overheating, or weird smells, you’re probably okay to drive it for now. Just keep in mind, it’s polluting more than it should be. If you live somewhere that does yearly emissions or smog checks, your car’s definitely not going to pass. Keep an eye on the check engine light too. If it starts flashing, that’s usually a sign of something more serious, like a misfire that could damage your engine. It’s fine to take short trips, just don’t forget about it or put it off too long. Try to get it checked out as soon as you can.
Can bad spark plugs cause P0420 Code?
A bad spark plug is different than a worn spark plug. A bad spark plug is caused by breakage or defect in the spark plug. And, in my experience, both, bad and worn spark plugs can cause a P0420 Code. They do this by not helping burn the gasoline properly, which leads to excessive unburnt gasoline hitting the Catalytic Converter and lowering it’s capacity to reduce pollution, even damaging it. This excess gasoline causes bad reading from the Oxygen sensors and the computer responds by turning on P0420 Code.