P0420 Code: Causes, Meaning, Symptoms (and How to Fix it)

There are several issues that can cause the Check Engine Light to come on. However, if you get the P0420 code using an OBD-II scanner, it could be a problem with your catalytic converter. P0420 is the usual trouble code. This article focuses on the symptoms, causes, and fixing cost of the P0420 code.

P0420 Code Definition

P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1).”

What is the P0420 Code Mean?

The P0420 code indicates the poor efficiency of the catalytic converter at Bank 1. This code may also be triggered due to a bad catalyst or a false alarm caused by a damaged oxygen sensor at Bank 1.

P0420 Code

Basically, Bank 1 and Bank 2 are the two sides of the engine. Bank 1 represents the side of the car’s engine containing cylinder No. 1, while Bank 2 represents the opposite side of Bank 1.

Bank 1 and Bank 2

The powertrain control module (PCM) of your vehicle gets information from different engine sensors. It uses this information to control engine functionality.

The PCM has two oxygen sensors. They are both placed near the catalytic converter; one is in front of the converter, and the other is behind it. These sensors are used to measure the efficiency of the catalytic converter.

Readings of the upstream O2 sensor should vary if the car is working at the optimal temperature or in the closed loop. This variation shows that the upstream O2 sensor is operating efficiently.

While in the case of the downstream O2 sensor, its reading shouldn’t be changed if your catalytic converter is working efficiently.

There may be a problem with the catalytic converter if the readings of both O2 sensors are the same. The reduction and fluctuation in the voltages (like the voltage of the upstream O2 sensor) of the downstream O2 sensor occur due to the high O2 level within the system. Due to this high oxygen level, the PCM will trigger the P0420, and your check engine light will start to illuminate on your car dashboard.

The efficiency of the catalyst can be reduced permanently or temporarily due to the improper fuel-air mixture, and this can be the reason for P0420.

Symptoms of P0420 Code

A car with the P0420 fault code can experience one of the following symptoms:

  1. Check engine light illumination
  2. The bad smell like rotten eggs coming from the exhaust pipe
  3. Reduced engine power
  4. Rough idle, misfires, hard shifting, and truncated driveability
  5. More fuel consumption than usual
  6. Smell like sulfur or rotten eggs
  7. Inability to accelerate quickly or travel at speeds greater than 30 to 40 mph
  8. Improper fuel-air mixture

Read Also: Causes of Check Engine Light Illumination 

Causes of the P0420 Code

A bad catalytic converter is the major cause of the P0420. Additionally, damaged oxygen sensors, intake leaks, faulty muffler deletions, exhaust system leaks, or improper placement of the catalytic converter may cause DTC P0420.

The primary causes of the P0420 code are listed below:

  1. A faulty downstream or upstream oxygen sensor
  2. Leakage in the exhaust system
  3. Damaged engine coolant temperature sensor
  4. Usage of the wrong type of fuel
  5. Bad exhaust pipe
  6. Loose or improper wiring of the oxygen sensor
  7. Damaged air-fuel sensor
  8. The rich or lean air-to-fuel ratio
  9. Faulty wirings
  10. Damaged powertrain control module (PCM)
  11. A leaky fuel injector
  12. Engine Misfiring, which damages the catalytic converter
  13. Bad catalytic converter
  14. Higher fuel pressure than usual
  15. Contaminated oil in the catalytic converter

Read More: P0455 Symptoms and Causes

How to diagnose the P0420 Code?

  • Read the trouble code with the help of an OBD2 scanner
  • Check the live data of the downstream (rear) O2 sensor, and the reading must be constant. Analyze the readings to check whether the rear O2 sensor is working efficiently or not.
  • Search for other trouble codes. There is a chance that these codes may be the reason for P0420.
  • Solve all the ignition problems, misfire issues, and fuel system faults.
  • Check for any damage and wear on the O2 sensor.
  • Take your car for a test drive and see if the downstream (rear) O2 sensor is working efficiently or not.
  • In the case of a faulty catalytic converter, replace the converter and update the PCM.
  • Inspect your PCM and replace or reprogram it if needed.

Common P0420 Code diagnosis mistakes

The following list contains the most common P0420 diagnosing mistakes:

  • Replacing the oxygen sensors before finishing the diagnostic procedure.
  • Always inspect for other DTCs in the system. The P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308 error codes represent the engine misfires and may lead to the P0420 error code. In this situation, try to resolve the misfire issue and again check if the P0420 is still here or removed.
  • The DTC’s P0171, P0172, P0174, or P0175, represent the engine running lean or rich. These errors can be due to the MAF sensor, high or low fuel pressure, a fault in the engine sensor, or a faulty fuel injector. First, try to resolve the air-fuel mixture issue and then check for the P0420 again.

What repairs can fix the P0420 Code?

  1. Changing the front or back O2 sensor
  2. Change the muffler or fix any muffler leak.
  3. Find and correct engine misfiring issues.
  4. Fix lean or rich fuel mixture
  5. Change or repair leaky fuel injectors
  6. Fix the exhaust manifold leaks or replace the exhaust manifold if needed.
  7. Identify and resolve any other associated error codes that the Power Control Module has saved
  8. Fix or replace the O2 sensor connectors
  9. Replace broken exhaust pipes or fix leaks in existing ones.
  10. Replacing the coolant temperature sensor
  11. Fix damaged wiring to oxygen sensors
  12. Catalytic converter replacement

Code P0420 Repair Cost

The repair cost of the P0420 code varies according to relevant part repair, vehicle model, and labor cost. If the P0420 code is triggered due to a faulty catalytic converter, your mechanic may cost from $480 to $1610 to fix it

The estimated cost of repairing the P0420 code is given below. The prices include parts and labor work at a workshop.

  • Catalytic Converter Replacement: 480$ to 1610$
  • Rear O2 Sensor Replacement: 130$ to 320$
  • Front O2 Sensor Replacement: 140$ to 320$

FAQ Section

How serious is the P0420 code?

P0420 code is not so serious. In most circumstances, this code won’t cause any serious issues for the engine of your automobile.

The worst thing that can happen in the case of P0420 is the blockage in the exhaust flow due to any broken internal part of the catalytic converter. But still, there is a very low probability of this happening.

Can a bad O2 sensor cause a P0420 code?

Yes. The normal catalyst monitor uses the downstream O2 sensor signal to calculate the efficiency of the catalyst, while the upstream O2 sensor signal is used as the reference point. Therefore, the P0420 code can be set when either the upstream or downstream O2 sensor goes bad.

However, the most common cause of this code is a defective catalytic converter.

How do I clear a DTC P0420?

Use a code reader or scan tool to clear the P0420 code temporarily. The main issue is that when the PCM of your runs its system self-test, it returns a code. In order to close the code and keep it closed, you must first find and resolve the underlying issues that triggered the code and then use a scan tool to clear the code.

Can I drive with a P0420 Code?

Yes, you can drive your car with code P0420, but experts don’t recommend it. Driving a vehicle with a P0420 can badly damage your vehicle’s catalytic converters. This issue may also damage other parts of your car, like the fuel lines and the exhaust system. That’s why you must regularly inspect your car’s catalytic converter to avoid further costly repairs.

What can damage the catalytic converter?

  • Misfires
  • Intake leaks
  • Bad powertrain control module
  • Oil consumption
  • A lean or Rich air-fuel mixture
  • Lean mixture
  • Exhaust leaks

What is the P0420 code on a Chevrolet?

When your Chevrolet is experiencing a p0420 code, it is usually caused by an exhaust leak before the catalytic converter. It may also trigger due to a bad catalytic converter or downstream (back) O2 sensor.

Chevrolet engines have been known to leak around the exhaust manifold. So, if you notice a strange noise when the engine is running, be sure to check it out.

Read More
  1. PCM Function, Symptoms, and Causes
  2. Bad O2 Sensor Working, Symptoms and Causes
  3. P0017 Code Symptoms and Causes
  4. P0453 Symptoms and Causes
  5. P0507 Symptoms and Causes

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