P0420 Code : Meaning Causes & How To Fix It

p0420 code

A variety of sensors are used in modern vehicles. These sensors work according to different prescribed needs. But sometimes these sensors can get damaged due to some reason or another.

A bad sensor becomes unable to send an output signal to the ECU, and a variety of sensor-related trouble codes appear, indicating a fault associated with the sensor. Similarly, vehicles have the P0420 trouble code, which indicates a problem with the catalyst system efficiency below limit bank1. In this post, we will learn in detail about the causes of the P0420 fault code and how to fix it.

What Is P0420 Code?

The P0420 code refers to a fault called ”Catalyst System Efficiency Below Threshold (Bank1)” in vehicles. This is a common OBD-II diagnostic code that indicates a problem with the vehicle’s catalytic converter not effectively reducing emissions.

What Does P0420 Code Mean?

The P0420 code in vehicles indicates low efficiency of the catalytic converter on Bank 1 or its poor performance. This causes the ECU to have some kind of problem receiving or functioning with the output signal from the catalytic converter.

The ECM uses two O2 sensors to measure the efficiency of the catalyst. These O2 sensors are usually located on the front or rear end of the catalyst in vehicles. This sensor sends a signal to the ECM about the efficiency of the catalyst. If the catalyst’s system functionality is reduced, it may trigger the P0420 code.

Causes of P0420 Code

The main causes of the P0420 code can include a bad or worn catalytic converter, exhaust leak, engine misfire, damaged wire connections, a faulty oxygen sensor, or engine failure.

These cause harmful emissions from the engine, which can result in increased levels of carbon monoxide and hydrocarbons in the exhaust.

1. Faulty Catalytic Converter

The most common cause of code P0420 in a vehicle is a catalytic converter failure. If there will be a decrease in efficiency in the catalytic converter system or the performance of the catalytic converter is weak, the O2 sensor in the catalytic converter may display code P0420 on the dashboard.

2. Exhaust Manifold Leak

If there is any leakage in the exhaust manifold of the vehicle. So the efficiency of the catalytic converter may decrease. Due to this, the O2 sensor sends a signal to the Engine Controller Module (ECM) of the vehicle to signal a decrease in the efficiency of the catalytic converter and the ECM may show the P0420 code because of the input command.

3. Engine Misfires

If for any reason the engine of the vehicle misfires. And up/down the engine rpm. In this case the catalytic converter fitted to the vehicle’s exhaust system may decrease in proportion to its efficiency. Because of this, the O2 sensor may show code P0420. Because when this happens, the O2 sensor starts sending the wrong signal.

4. Incorrect Catalytic Converter Fitting

In vehicles, the exhaust catalytic converter is fitted by the manufacturer at the correct location and correctly. But later, if the exhaust assembly is opened again due to some reason, if it is fitted again in the wrong place or incorrectly. So the O2 sensor on the catalytic converter is not properly sensing the efficiency of the catalytic converter, this could also be the main reason for the P0420 code.

5. Using Wrong Fuel

The fuel used in vehicles is purified by a refinery, in which a variety of unnecessary elements are removed from the fuel. However, sometimes leaded fuel is accidentally used in vehicles, which disrupts the combustion process taking place in the engine’s combustion chamber.

This reduces the efficiency of the catalytic converter installed in the exhaust system of the engine. Because of this, the O2 sensor in the catalytic converter may show the P0420 code.

6. Faulty Coolant Temperature Sensor

Sometimes the coolant temperature sensor installed in the vehicle gets damaged, due to which the wrong signal of engine temperature is received by the ECM. Due to this, the ECS unit installed in the vehicle may also show the P0420 code.

7. Faulty Oxygen Sensor

A malfunction of the oxygen sensor in the vehicle’s catalytic converter can also trigger the code P0420. Because this O2 sensor is unable to measure the efficiency readings of the catalytic converter, the ECM does not receive an incorrect signal or any signal of any kind in the reading of the efficiency of the catalytic converter.

8. Damage Wires & Loose Connection

The O2 sensor in the vehicle’s catalytic converter transmits an output signal to the ECM through a series of wires. But sometimes, due to some reason, the sensor wire connections become damaged or loose. Due to this, the O2 sensor is not able to send an output signal to the ECM and the ECM may display code P0420 due to a signal error.

9. Fuel Injectors Leakage

If there is any kind of fuel leakage in the fuel injector in the engine, then the efficiency of the catalytic converter may decrease. Due to which P0420 code error may occur.

10. Rich or Lean Air-Fuel Ratio

If the flammable fuel mixture in the combustion chamber of the engine changes or malfunctions in the ratio of the fuel and air mixture, the combustion process in the combustion chamber may be interrupted, due to which the efficiency of the catalytic converter may decrease. Because of which O2 sensor may show P0420 code error.

11. Contaminated Catalytic Converter

When contaminated oil starts accumulating in the catalytic converter. Then the catalytic converter would be able to extract the right amount of exhaust gas. O2 sensors may show a P0420 code error due to decreased catalytic converter efficiency.

Symptoms Of P0420 Fault Code

Common symptoms of a P0420 fault code may include the check engine light illuminating, decreased engine power, decreased fuel efficiency, engine rough idle, RPM down, blue smoke from the exhaust, and the inability to accelerate at slow speeds.

Let us understand these symptoms in detail so that you do not get confused after seeing symptoms similar to this code.

1. Engine Check Light

The main symptom of a P0420 code error could be the check engine light. If there is any error in this code, the check engine light in the cluster meter of the vehicle turns on.

2. Loss of Engine Power

Often, the engines of some vehicles exhibit a slight decrease in idle speed. But if the error code P0420 is coming from the vehicle for some reason, the engine of the vehicle will show less power even when it is hot. These symptoms could also be due to the P0420 code error.

3. Decrease Fuel Efficiency

Error code P0420 in the vehicle causes the vehicle engine to reduce its power performance. Due to this, it consumes more fuel and suddenly reduces the fuel mileage. These signs may also be code P0420 in vehicles.

4. Engine Rough Idle

Error code P0420 may result in the engine starting in a rough idle position. And keeps on increasing or increasing the rpm at idle position. The engine being started at rough idle can also result in code P0420.

5. RPM Down When Turn On The AC

Whenever you get the P0420 code error inside the vehicle, the engine power is weak and with this, when the vehicle’s AC is turned on, the engine’s RPM goes down from its normal state, due to which the engine of the vehicle starts beating. and may misfire. These symptoms could also be due to the P0420 error code.

6. Blue Smoke From Exhaust

The error code P0420 can cause blue and raw smoke to appear from the exhaust pipe when the engine is started. Due to this code the ECM is unable to determine the firing order for the fuel mixture at the right time, which disrupts the combustion process inside the combustion chamber and blue smoke can be seen coming out of the exhaust pipe due to the fuel not burning properly.

7. Inability To Accelerate in Slow Speed

Whenever the vehicle is moving at a slow speed of 30 to 40 mph, it is difficult for the engine to accelerate. Due to this, the engine of the vehicle can show misfire and poor performance and after a long time, it slowly picks up speed. This can also be a symptom of error code P0420.

How Can A Mechanic Diagnose A P0420 Code Error?

A mechanic uses a digital scanner tool to diagnose the P0420 code. The OBD scanner can check the cause of the P0420 code and the performance of sensor parts and give the exact address of the cause of the malfunction.

  • A mechanic uses an OBD-II scanner to obtain the PCM’s recorded error codes.
  • Views real-time data from upstream oxygen sensors. The reading downstream from the oxygen sensor should be stable.
  • Check to see that the downstream (rear) oxygen sensor is functioning properly.
  • Works on the repair of fuel system problems, ignition problems, or misfires as needed.
    Checks the rear oxygen sensor to see if it is damaged.
  • Tests the car by looking at freeze frame data to check the functionality of the downstream (rear) oxygen sensor.
  • If the catalytic converter is broken, replace it and check the ECU for any available updates.
  • Replacing the catalytic converter will require an update to the ECU, which the mechanic will update appropriately.

How To Fix P0420 Code?

First, scan the vehicle system with a digital scanner tool to find the cause of the P0420 code. There could be any of these reasons, which can be fixed by checking step by step.

  • When the P0420 code appears, first reset this code using the digital scanner and drive for 15 to 20 minutes. If error code P0420 occurs again, you can follow the next steps.
  • Then check the oxygen sensor reading to see if the O2 sensor on the catalytic converter is working properly. If any defect is found in the oxygen sensor, replace the oxygen sensor with a new one.
  • Next, check for leaks in the exhaust manifold. If a leak is found in the exhaust manifold, repair it or replace the exhaust manifold with a new one.
  • Clean the catalytic converter, and in case of major failure, you will have the option of replacing the catalytic converter with a new one.
  • Repair the exhaust pipe leakage or replace the exhaust pipe with a new one due to excessive leakage or breakage.
  • Check the O2 sensor wiring and connectors, and repair the wiring if damaged.
  • Replace the engine coolant temperature sensor with a new one, and also check the thermostat valve.
  • Repair fuel injector leaks and replace the injector O ring with a new one if leaking occurs.
  • Diagnose any causes of engine misfire.
  • Update the faulty ECM or replace it with a new one.

Mechanic’s Mistake When Diagnosing Code P0420

Replacing the oxygen sensor is one of the main mistakes mechanics make before finishing the diagnostic process. If another component is at fault, replacing the oxygen sensor will not solve the P0420 error code problem. For this reason, many reasons for the occurrence of this code can be rectified by examining them one by one.


Q. How serious is code P0420?

Driving with the code P0420 is considered a common cause. Because usually when this trouble code comes, the symptoms are not given much attention. But driving the vehicle for a long time with this code can damage some or other parts of the engine and with that the catalytic convertor can go bad at most. Changing it for a new one can be an expensive option.

Q. Can the P0420 code be fixed at home?

These codes can be fixed at home by examining the common causes of them. After correcting the common causes of code P0420, disconnect one battery terminal for 15 to 20 minutes, due to which the engine light can be reset.

Q. How much can it cost to fix a P0420 code?

The cost of fixing this code depends on the model of the vehicle and the defect in the parts. The labor cost for a mechanic to diagnose the vehicle and fix the code can be anywhere from $70 to $150, and the cost of replacing a new part for a P0420 code fault can range from $50 to $1900.

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