The P0420 code signals a low catalyst system efficiency. This code suggests that the oxygen levels are below the desired threshold (Bank 1), which most often results from problems with your car’s exhaust or fuel systems. It can sometimes be tricky to diagnose.
The catalytic converter is a component of the vehicle’s exhaust system. It works by oxidizing the remaining unburned fuel and carbon monoxide (CO) from the combustion process into carbon dioxide (CO2) and water (H2O). It also reduces nitrogen oxides (NOx) into nitrogen.
The converter uses two oxygen sensors – one on the upstream and the other on the downstream. If these sensors show similar readings, something’s wrong. That’s when the Powertrain Control Module (PCM) triggers the P0420 code, causing the check engine light to come on, alerting you that you have a catalyst problem.
What Does the P0420 Code Mean?
Diagnostic trouble code (DTC) P0420 stands for “P0420 Catalyst System Efficiency Below Threshold (Bank 1).”
It indicates that the exhaust system’s oxygen levels are below the required threshold in Bank 1, which can damage the engine’s exhaust and fuel system.
Bank 1 refers to the side of the engine that houses the number one cylinder. The opposite side of the engine is Bank 2.
A Closer Look at the P0420 Code
A catalyst monitor or catalytic converter monitors the converter’s ability to reduce harmful emissions using oxygen. Technically, the sensor that monitors catalyst efficiency is measuring the oxygen storage capacity of the catalyst in that converter.
When the catalytic converter is performing well, it uses oxygen from the exhaust gas to break down pollutants. The PCM compares the output of upstream and downstream oxygen sensors to determine the level of oxygen entering and leaving the converter.
Ideally, there should be little to no unburned oxygen after the process. What this means is that the downstream (catalyst monitoring) sensor should have a very lazy signal compared to the upstream sensor.
When combustion occurs in each cylinder, pulses of oxygen hit the upstream oxygen sensor. The oxygen sensors generate voltage signals that fluctuate with the pulses.
When the catalytic converter works, the downstream oxygen should detect little to no oxygen. Its voltage signal remains consistent to indicate the converter has depleted the oxygen.
The voltage must be 650 millivolts to indicate a low oxygen level. If the voltage is lower than that points to a high oxygen level, which means the catalytic converter was inefficient.
Which Cars Can Trigger the P0420 Code?
The catalytic converter has been a required component on all cars since 1975.
Code P0420 applies to all vehicles made after 1996 and some made as early as 1994, as catalytic converter monitoring became standard in cars to comply with the OBD2 element of U.S. National Emissions Standards.
Symptoms of the P0420 Code
While the P0420 code may not always result in noticeable issues with vehicle performance, it is important to recognize the potential symptoms. Common signs include:
- Illuminated Check Engine Light: The most obvious symptom of the P0420 code is the activation of the check engine light.
- Decreased Fuel Efficiency: A failing catalytic converter can cause your engine to work harder, leading to reduced fuel economy.
- Reduced Engine Performance: You may notice a loss of power or sluggish acceleration.
- Unusual Exhaust Odor: A failing catalytic converter may produce a sulfuric or rotten egg smell from the exhaust.
- Failed Emissions Test: Vehicles with a P0420 code are likely to fail emissions testing due to excessive pollutants in the exhaust.
Common Causes of the P0420 Code
The P0420 code can be caused by a variety of factors, many of which are related to the catalytic converter or other components in the exhaust and fuel systems. Some of the most common causes include:
- Faulty Catalytic Converter: Over time, the catalytic converter can become clogged, damaged, or worn out, reducing its efficiency.
- Malfunctioning Oxygen Sensors: The upstream (pre-catalytic) and downstream (post-catalytic) oxygen sensors monitor the exhaust’s oxygen content. A faulty sensor can send incorrect readings to the ECM, triggering the P0420 code.
- Exhaust Leaks: Leaks in the exhaust system can disrupt the readings from the oxygen sensors, leading to incorrect efficiency calculations.
- Engine Misfires: Misfires can cause unburned fuel to enter the exhaust system, which can damage the catalytic converter over time.
- Rich or Lean Air-Fuel Mixture: Problems with the air-fuel ratio can lead to incomplete combustion, affecting catalytic converter performance.
- Oil or Antifreeze Contamination: Oil or coolant leaks into the exhaust system can damage the catalytic converter.
- ECM Software Issues: In rare cases, a malfunction or outdated software in the ECM can result in erroneous diagnostics.
How Difficult Is It to Inspect a P0420 Code?
Before getting started with any repairs, you’ll need to properly diagnose the P0420 code. Most service professionals will use an OBD-II scanner to retrieve the particular code the Power Control Module (PCM) is showing.
If the voltage of the downstream oxygen sensor decreases and fluctuates like the upstream oxygen sensor, this is a certain indication that the oxygen levels are too high and the PCM will throw the P0420 code.
However, if there aren’t any visible issues with the upstream and downstream oxygen readings, the mechanic will then check engine misfires, ignition problems and even the fuel system to check for any malfunctions. The mechanic may also check the rear O2 sensor to gauge any excess wear or damage on the part.
An experienced professional will test drive the vehicle and monitor the freeze-frame data. This allows them to determine whether the rear O2 sensor is working properly.
Finally, we also suggest updating the PCM to the latest version just in case there’s something wrong with it. It’s not uncommon that an out-of-date PCM is behind your vehicle’s issues.
How Do I Fix the Error Code P0420?
P0420 is a typical error code caused by exhaust leakage. First, examine the exhaust system for evident damage. Welding could be a simple and inexpensive solution.
However, if your exhaust is in bad general condition, it will most likely only be a temporary repair. After that, it could come down to the necessity for a new exhaust system, exhaust manifold, etc.
Here’s a step-by-step guide to fix the P0420 error code
Step 1 – Fix other codes
First, resolve any other codes that may have come up besides the P0420; especially troublesome is the P0300 series of cylinder misfire codes. A P0300 code could be related to something in the ignition (spark plug or coil), the fuel injector, compression, or fuel contamination.
Step 2- Check for and resolve exhaust system leaks
Pressurize the exhaust system with compressed air and spray soapy water around suspected areas. Once identified, fix it using exhaust tape, epoxy, gasket replacement, tightening loose bolt/s, or cutting a portion out and joining it together with new pipe and clamps.
Step 3- Check for cylinder imbalance
If one cylinder is running lean and the other cylinders are running rich, this indicates that the engine has an imbalance in power output. This imbalance can create excessive emissions by simultaneously raising hydrocarbon and nitrous oxide output.
Most advanced diagnostic scan tools can run a cylinder imbalance test, but another way to check for cylinder imbalance is to scope the ignition pattern. In general, cylinders with short burn lines indicate lean cylinders.
Step 4- Check your spark plugs
Another cause of high emissions is worn-out or faulty spark plugs. Many modern vehicles come equipped with long-lasting iridium spark plugs to meet stringent emission durability requirements of 80,000 or more miles.
These long-lasting iridium spark plugs sometimes get replaced with spark plugs made of other materials and may not provide the same length of service life. As the spark plug starts to fail, emissions can increase.
While on this, make sure to clean your catalytic converter efficiently.
How do you fix code P0420 on the Honda Accord?
Here are some steps to fix a P0420 code Honda Accord.
- First, remove the O2 sensor and pour 4-6 ounces of seafoam into the opening, replace the sensor, and leave it overnight. Reset the code and then continue driving.
- Increase the gap between the oxygen sensor and the converter using a spark plug non-fowler.
- The most typical fix for the error code is to replace the catalytic converter.
- If required, replace the oxygen sensors. But it won’t always be the case.
- Replace the oxygen sensor’s wire or connectors.
- Then, clean the catalytic converter.
- Change the air intake hose.
How do I fix my Nissan P0420 code?
Here are some steps you can follow to fix a P0420 code Nissan.
- Examine the exhaust and ensure there are no leaks. You can locate the upstream oxygen before the catalytic converter.
- Use a spark plug non-fowler to create space between the O2 sensor and the catalytic converter.
- Then, clean the catalytic converter. Reduce the gasoline level to one-quarter of a tank. Fill the gas tank with the cleaner. Drive the car for 150 miles or run the engine for 30 minutes at around 200 rpm.
- Do a replacement for the catalytic converter.
- Replace one or both O2 sensors as required.
- If needed, replace the O2 sensor’s wire or connectors.
How Much Does It Cost To Fix?
Many customers who’ve been unfortunate enough to see the P0420 or P0430 error code appear on their vehicle will report a hefty repair bill. Usually, the labor cost to replace a catalyst is only a couple of hours on average. The real expense is in the part itself.
The cost of the replacement catalyst will vary from vehicle to vehicle. RepairPal estimates catalytic converter replacement between $2,200 and just over $2,300. We’ve seen the prices go as high as $3,200 for the integrated catalytic converters. On the Porsche 911, a catalytic converter replacement will go for over $3,200; you’ll generally pay more for this part on high-end models.
Alternatively – and much more affordably – catalytic converter fuel additive treatments can cost between $15 and $50. These chemicals are added to your fuel tank.
Additionally, catalytic converters may be covered under special emissions warranties, which can cover some specific emissions components for 96 months or up to 80,000 miles. Check with your dealership to see if your vehicle is covered.
The P0420 error code can sometimes be remedied by using a higher octane fuel or chemical treatments. We recommend attempting some of the cheaper alternatives before pricing a replacement.