Understanding P0420 Catalyst System Low Efficiency – 13.2%: A Comprehensive Guide

Understanding the P0420 Code

Detailed Explanation of the P0420 Code

The P0420 code indicates that the oxygen levels in the exhaust system on Bank 1 are not meeting the necessary threshold, which can adversely affect the engine’s exhaust and fuel systems. Bank 1 refers to the side of the engine that includes the number one cylinder. The opposite side is Bank 2.

The illustration provided demonstrates the functionality of each catalyst and how a properly working catalyst appears when monitored by upstream and downstream sensors. If the downstream sensor’s readings are similar to those of the upstream sensor, a P042x code, such as P0420 or P0421, is triggered.

Vehicles Prone to Triggering the P0420 Code

Since 1975, catalytic converters have been a mandatory component in all cars. The P0420 code applies to vehicles manufactured after 1996 and some models as early as 1994, as catalytic converter monitoring became a requirement for compliance with OBD2 regulations under the U.S. National Emissions Standards.

P0420 Code in Honda Vehicles

The P0420 code may also appear in some Honda models. For instance, in a 2013 Honda Accord LX 2.4L, the code can be triggered by:

• WU-TWC degradation
• Electrical noise on the secondary HO2S (sensor 2) line
• Loose connection in the secondary HO2S (sensor 2) connector
• Changes in the exhaust system (non-genuine parts)

For this vehicle, Honda notes that if a malfunction is detected during the first drive cycle, a Pending DTC is stored in the PCM memory. If the malfunction occurs again in the next (second) drive cycle, the MIL illuminates and a Confirmed DTC, along with freeze data, is stored. If the malfunction does not recur in three consecutive trips where the diagnostic runs, the MIL is cleared. The MIL, Pending DTC, Confirmed DTC, and freeze data can be reset using a scan tool or by disconnecting the battery.

Potential Causes of the P0420 Code

• Ineffective catalytic converter
• Faulty oxygen sensor
• Damaged wiring
• Loose connections
• Exhaust leak
• Damaged exhaust manifold
• Faulty spark plug
• High fuel pressure
• Fuel injector leak
• Using the wrong type of fuel
• Outdated PCM software

Symptoms Associated with the P0420 Code

• Check engine light illumination
• Failed emissions test
• Exhaust pipe emitting a rotten egg smell
• Reduced engine power
• Difficulty accelerating

Diagnosing the P0420 Code

The P0420 code is a generic powertrain code that can appear on various vehicles, and the diagnostic process can vary depending on the make and model. Mechanics typically follow these steps to diagnose the code:

1. Establish a Baseline: Record the freeze frame data when retrieving the code, which serves as a reference for tests.
2. Address Issues Affecting the Catalyst: Resolve misfires, ignition problems, and fuel or intake issues that could trigger the P0420 code to rule them out.
3. Test Drive Under Freeze Frame Conditions: Verify if the rear oxygen sensor mimics the front one and/or produces voltage below the required threshold. If these conditions are confirmed, the catalytic converter is likely defective.

Mechanics may perform additional tests based on their findings. For DIY diagnosis, research the necessary tools and ensure you have them before starting.

Fixing the P0420 Code

Fixing the P0420 code can be challenging as the solution depends on the cause. Without the proper tools and expertise, it can be difficult to address the issue. It’s often best to have a professional handle the repair. However, if you choose to do it yourself, rely on vehicle-specific repair information from sources like ALLDATA or Chilton for detailed guidance.

Catalytic Converter FAQs

Lifespan of a Catalytic Converter

A catalytic converter can last up to 10 years or more, but they are designed to last as long as the vehicle.

Understanding Catalysts

Function of Catalytic Converters on Exhaust Gases

Catalytic converters have a fine, honeycomb structure with tiny channels coated with catalysts that detoxify the exhaust gases passing through them. These catalysts are typically made of rare and expensive metals such as palladium, platinum, and rhodium, which is why catalytic converter theft is common.

When the catalysts reach operating temperature, they break down harmful gases. The catalyst nearest the engine, known as the “light-off catalyst,” separates NOx into nitrogen and oxygen components. This is the only catalyst monitored by the PCM. Another catalyst, usually farther from the engine, adds oxygen to HC and CO to convert them into CO2.

Additional Notes on the P0420 Code

When addressing the P0420 code on your own, avoid replacing parts without identifying the exact cause. Rule out other potential causes of catalytic system inefficiency before replacing O2 sensors or the catalytic converter. Some car owners mistakenly replace a functioning catalytic converter due to wiring or software issues, wasting money and not resolving the code.

P0420 Code FAQs

Clearing a P0420 Code

To clear this trouble code, identify and address its underlying cause.

Impact of a Faulty O2 Sensor on the P0420 Code

Yes, a faulty O2 sensor can cause the P0420 code by making the catalytic converter inefficient. The catalyst monitor uses signals from the downstream O2 sensor to determine catalyst efficiency and uses the upstream O2 sensor as a reference point. If either sensor fails, it can trigger the P0420 code.

P0420 Code in Nissan Vehicles

In newer Nissan models, the P0420 code means “Three-Way Catalyst Malfunction” rather than “Catalyst System Efficiency Below Threshold (Bank 1).” However, it still indicates a decline in the catalytic converter’s performance.

Testing O2 Sensors for a P0420 Code

To test if faulty O2 sensors triggered the P0420 code, force the air/fuel mixture into rich and lean conditions. To make the mixture rich, add propane to the engine’s air intake and increase RPMs to prevent stalling. Monitor the oxygen sensors with a scan tool to see if they detect changes. To create a lean condition, disconnect a large vacuum hose and track whether the oxygen sensors’ signals respond accordingly. If the sensors’ readings change, they are working correctly.