P0420 Code – Meaning, Causes, Symptoms, And How To Fix

The P0420 code is a common trouble code. It indicates an issue with the catalytic system. Learn the P0420 code meaning, causes, and symptoms.

The role of the catalytic converter is to transform harmful gases generated in the combustion process into less harmful gases, thus reducing the emissions discharged through the tailpipe.

The performance of the catalytic converter is monitored by the engine control module (ECM). Whenever the ECM observes an issue with the catalytic converter located at Bank 1, it activates the diagnostic trouble code P0420. This article explains the P0420 code meaning, causes, and how to fix it.

P0420 Code Definition

P0420 Code – Catalyst System Efficiency Below Threshold (Bank 1).

In terms of engine layout, Bank 1 and Bank 2 refer to the engine sides. Bank 1 corresponds to the engine’s side that has cylinder 1, while Bank 2 indicates the engine side containing cylinder 2.

What does the P0420 Code Mean?

The P0420 code indicates that the engine control module (ECM) of the vehicle detects that the Bank 1 catalytic converter is not performing efficiently.

The catalytic converter of a vehicle plays a crucial role in minimizing harmful pollutants that pose risks to both human health and the environment. It converts up to 90 percent of noxious emissions into less damaging gases.

The healthy and efficient performing catalytic converter has a direct impact on the vehicle’s overall performance and health. By utilizing intricate platinum and gold meshes, the catalytic converter filters exhaust emissions, leading to reduced harmful gases in the exhaust discharged through the tailpipe.

The catalytic converter usually contains two oxygen sensors. One is located upstream, before the converter, while the other is located downstream, after it.

When your vehicle is functioning at its optimal temperature and functioning in a closed loop, the upstream O2 sensor’s readings should display fluctuations. Meanwhile, the downstream O2 sensor’s readings shouldn’t be changed if the converter is functioning correctly.

However, if both O2 sensors exhibit similar readings, it suggests the catalytic converter isn’t performing according to the requirements. When the downstream O2 sensor’s voltage drops and its pattern of fluctuations mirror that of the upstream sensor, it indicates an increase in the oxygen level. As a result, the Engine Control Module (ECM) stores the P0420 trouble code and activates the check engine light.

Symptoms of P0420 Code

Illuminated check engine light
Too lean or too rich air-fuel mixture
Rotten egg smell
A reduction in engine power
Unusual noises
Hard shifting
Poor fuel efficiency
Failed emission tests
Rough idle
Failure to drive the vehicle at speeds more than 30 to 40 mph

Causes of P0420 Code

The engine control module of a vehicle may trigger the DTC P0420 due to one or more of the below-given reasons:

Damaged muffler
Vacuum leaks
A leak in the exhaust manifold
Engine misfiring
Oil leakage into the catalytic converter
A leak in the tailpipe
Catalytic converter failure
An issue with the coolant temperature sensor
Faulty downstream oxygen sensor
An issue with the wiring and connection connected to the oxygen sensors
An issue with the upstream O2 sensor
Damaged or dirty fuel injector
Faulty spark plugs
Use of poor-quality fuel

Read More: Bad O2 Sensor Symptoms and Causes

How to diagnose the P0420 code?

When the vehicle triggers the P0420 code, it is very important to fix it promptly. Driving with this code for a long time may badly damage your engine. Before diagnosing the issue, you have an OBD-II scanner compatible with your vehicle. You should also have a voltmeter or multimeter to check the voltage of different parts.

Follow the below-given steps to diagnose the P0420 code:

Begin by ensuring that the P0420 code is the sole engine code stored in the ECM. You can use an OBD-II for this process.
Address other related trouble codes before focusing on the P0420.
Erase the ECM memory and drive your vehicle for about 14 minutes. Monitor whether the code reappears if it does not, repeat this method a few times to confirm. If the P0420 code reappears, a deep inspection is needed.
Physically inspect your exhaust system for leaks, rust, holes, or damage. Remember to examine the exhaust manifold.
Check the voltage output from the oxygen sensors by using a scan tool. In many models, the normal readings of the upstream oxygen sensor vary between 1 and 0.9 volts. While the reading of the downstream sensor should remain steady (i.e., around 0.6V). If the readings are significantly skewed or erratic, there is something wrong with the catalytic converter. Consult your vehicle’s service manual to find the exact voltage value for your O2 sensor.
Inspect the fuel and ignition systems for damage.
Examine the long-term and short-term fuel trim values to check if the engine is operating lean or rich.
Properly inspect the catalytic converter for damage.
Check the ECM for damage. Also, check the software of the ECM.

Common P0420 Code Diagnosis Mistakes

There are many mistakes that can be made while fixing the P0420 code. The following are the most common mistakes that can occur while diagnosing the P0420:

Not utilizing professional diagnostic tools while addressing the P0420 code
Skipping basic checks
Changing the catalytic converter without inspecting the exhaust system, O2 sensor, and electrical wires
Overlooking historical Information
Not addressing the issues related to O2 sensors
Ignoring engine performance problems
Depending only on the code without considering other symptoms
Overlooking exhaust leaks
Ignoring the necessary software updates
Not addressing other related codes
Not clearing the codes after fixing the issues

How serious is the Engine Code P0420?

The P0420 code is generally not a significant safety concern for driving. It’s suspected to result in severe issues related to driving performance. But it’s worth noting that your vehicle might be releasing higher levels of harmful emissions due to the underperformance of the catalyst system on Bank 1. This, in combination with the illuminated check engine light, could potentially lead to difficulties in passing emissions tests.

While P0420 doesn’t pose an immediate threat, it’s advisable to promptly address the issue. Delaying repairs for a long time isn’t recommended. It’s generally safe to continue driving with the engine code P0420, but it is best not to drive for a long time.

If your vehicle starts to exhibit concerning behaviors or if you notice other engine codes, significant warning signs, or unusual driving experiences, it’s safer to address the problem sooner rather than later.

What repairs can fix the P0420 Code?

To fix the diagnostic trouble code P0420, you need to repair one or more of the following parts:

Clean your catalytic converter if it is clogged
Repair leaks in the exhaust manifold
Replace or repair the leaked or damaged muffler
Repair or replace the damaged exhaust pipes
Fix the vacuum leaks
Change the faulty engine coolant temperature sensor
Replace the faulty catalytic converter
Replace the damaged downstream or upstream oxygen sensor
Fix or replace O2 sensor connectors
Repair the damaged electrical wires to the oxygen sensor
Clean or replace the malfunctioning fuel injectors
Fix the issues related to ECM
Diagnose and rectify any misfires
Clear the codes and take the vehicle for a test drive

How to Prevent P0420 Code?

Follow the below-given control measures to prevent the P0420 code:

Ensure regular maintenance of your fuel system and ignition system
Ensure O2 sensors maintenance
Use good quality fuel
Avoid ignoring minor repairs
Avoid aggressive driving

What could damage the catalytic converter?

The catalytic converter of your vehicle plays a crucial role in reducing the primary pollutants resulting from the combustion of the air-fuel mixture: hydrocarbons, carbon monoxide, and nitrogen oxide (NOX). The converter achieves this result by transforming these hazardous elements into less harmful compounds, promoting better air quality and safeguarding our well-being.

A catalytic converter may be damaged due to one or more of the below-given reasons:

Oil leakage in the catalytic converter
Misfires
Vacuum leaks
Exhaust leaks
Lean mixture
Rich mixture
Physical impacts
Use of wrong quality fuel

Repair Cost of P0420 Code

The repair cost of the P0420 code varies according to the labor cost and the severity of the issue. The repair or fixing cost of the P0420 code is given below according to the replacement cost of the relevant part:

Parts	Cost
Upstream Oxygen Sensor Replacement	$140 to $320
Downstream Oxygen Sensor Replacement	$140 to $320
Catalytic Converter Replacement	$480 to 1600

FAQ Section

Can I drive with P0420 Code?

Yes. It is possible to drive with a P0420 code; it’s not recommended. When your vehicle triggers P0420, it means something is wrong with the catalytic converter, which can result in reduced fuel economy and heightened emissions. In some instances, continuing to drive with a malfunctioning catalytic converter could potentially cause harm to other engine components. If you’re facing the trouble code P0420, it’s advisable to address it promptly.

How do I fix P0420 exhaust leaks?

P0420 is a common trouble code often triggered by exhaust leaks. As a starting point, inspect your exhaust for visible damages. If it is damaged, weld it properly. However, if the exhaust system is in poor overall condition, welding might only serve as a temporary solution. In such cases, the eventual requirement for a new exhaust manifold or exhaust system might be necessary.

What can cause the catalytic converter failure?

Your catalytic converter may become faulty due to different factors, such as physical impacts, poor quality fuel, excessive oil consumption, or engine misfires.

Which oxygen sensor causes the P0420 code?

The P0420 error code is triggered when both the downstream and upstream O2 sensors observe an issue with the catalytic converter’s performance. Essentially, the rear O2 sensor identifies that some air going into the converter is not undergoing complete combustion.

Can a bad O2 sensor cause a P0420 code?

A P0420 code commonly signifies catalytic converter issues, though a faulty oxygen sensor may also trigger it. The downstream oxygen sensor oversees catalyst efficiency; its failure may lead to a P0420 code. Conversely, a malfunctioning upstream oxygen sensor might cause engine lean conditions, potentially harming the catalytic converter.

Can a dirty fuel injector cause P0420?

Yes, a dirty fuel injector may trigger the P0420 code if it affects the combustion process. In such cases, replacing the faulty fuel injector and the shared fuel pipe could be necessary.

Can P0420 cause a car not to start?

When the Engine Control Module (ECM) logs the P0420 code, it could potentially lead to difficulties starting the vehicle. This is due to the downstream O2 sensor’s voltage dropping and displaying fluctuations similar to the upstream O2 sensor, particularly when the amount of oxygen is exceptionally high.

How long does it take for P0420 to reset?

The ECM of your car clears the codes itself after a set number of cycles, which involve turning the ignition on and off. This reset process typically occurs itself after approximately 10 to 20 cycles.

How hard is it to fix the P0420 code?

The fixing process of the P0420 code varies based on the underlying issue. Many issues can be performed by individuals at home. Basic fixes might involve tasks such as changing a faulty oxygen sensor or addressing damaged wires and connectors. However, certain repairs are more complex, requiring the expertise of a mechanic and possibly a visit to an auto repair shop.

Written By

Engineer Rahela

Rahela is the owner and main author of Mechanicalspecial. She is an automotive expert writer. She have over four years of experience in the automotive writing niche and have completed over 700 pieces of content from 40 projects. She have good knowledge and skill in vehicle repairing, all mechanical work, car upgrades and maintenance.