P0420 Code Symptoms and Causes | How to Fix P0420 Code?

There are many issues that could trigger the check engine light to illuminate. However, if you use an OBD-II scanner and get a P0420 code, you may be having issues with your catalytic converter. P0420 is one of the most common error codes. This article mainly explains the symptoms and causes of the P0420 code.

P0420 Code Definition

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

What does the P0420 Code Mean?

The P0420 code indicates that the PCM has detected underperformance of the catalytic converter’s efficiency on Bank 1. It can either be caused by a faulty catalyst or a false alarm caused by a faulty O2 sensor.

Bank 1 refers to the side of the engine that houses the number one cylinder. The opposite side of the engine is Bank 2

P0420 Code

The powertrain control module (PCM) uses two O2 sensors, one in front of the catalytic converter and one behind it, to measure its efficiency.

If the upstream oxygen sensor is working properly, its readings should fluctuate from when the car is at operating temperature and when running in a closed loop. If the downstream oxygen sensor is working properly and there isn’t an issue with the catalytic converter, its readings should remain steady.

When the oxygen sensors have similar readings to each other, it indicates that the catalytic converter is not working as intended.

If the voltage of the downstream oxygen sensor decreases and begins to fluctuate like the upstream oxygen sensor, it means the oxygen levels are too high, and the powertrain control module (PCM) will store the P0420 trouble code and turn on the check engine light.

An incorrect air-fuel mixture can also temporarily or permanently reduce the catalyst’s efficiency, resulting in a PO420 code.

Causes of the P0420 Code

The most common cause of the P0420 code is a bad catalytic converter. It can also be caused by a damaged muffler delete, intake leak, exhaust system leak, faulty oxygen sensor, or incorrect placement of the catalytic converter.

 Following are the major causes of the P0420 code:

  • Damaged catalytic converter
  • Increased fuel pressure
  • Fuel injector with a leak
  • Oil contamination in the catalytic converter
  • Damaged exhaust pipe
  • Incorrect placement of the catalytic converter
  • Misfires (damaging the catalytic converter)
  • Faulty wirings
  • Damaged upstream or downstream oxygen sensor
  • A leak in the exhaust system
  • Faulty engine coolant temperature sensor.
  • The wrong type of fuel (leaded fuel instead of unleaded)
  • Oxygen sensor wiring that is not properly connected.
  • Faulty air-fuel sensor
  • Rich or Lean air-fuel ratio (the root cause of converter failure)
  • The faulty powertrain control module

Read More: P0455, P0457

Symptoms of P0420 Code

You may notice the following symptoms if you drive a vehicle with a P0420 trouble code:

  • Illuminated check engine light
  • Lack of power from the engine
  • Slow acceleration or inability to increase speed over 30-40mph
  • Poor fuel economy
  • Rotten egg or a sulfur smell
  • A rotten egg smell coming from the exhaust pipe
  • Misfires, a rough idle, hard shifting, and overall reduced driveability
  • Air-fuel mixture imbalance

Read More: Causes of bad PCM

How to diagnose the P0420 Code?

  • Use an OBD-II scanner to retrieve trouble codes stored by the PCM.
  • View the live data of the downstream (rear) oxygen sensor. The downstream oxygen sensor voltage reading should be steady. Make a determination as to whether the downstream (rear) oxygen sensor is working properly.
  • Diagnose any other codes that could be causing the P0420 trouble code.
  • Repair misfires, ignition problems, and/or fuel system problems if necessary.
  • Inspect the rear oxygen sensor for damage and/or excessive wear.
  • Test drive the vehicle while viewing the freeze frame data to determine whether or not the downstream (rear) oxygen sensor is working properly.
  • Check for available updates of the PCM if the catalytic converter is faulty. After the catalytic converter is replaced, updates to the PCM will be required.

Common P0420 Code diagnosis mistakes

The most common diagnosis mistakes of P0420 are given below:

  1. Replacing the oxygen sensors before completing the diagnostic process.
  2. Check for any other trouble codes with your diagnostic scanner. If you find codes like P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308, you have a misfiring engine, and the misfires may cause the P0420 error code. In this case, you should fix the misfires first, clear the codes and see if the P0420 code comes back.
  3. If you find DTC codes like P0171, P0172, P0174, or P0175, it means that your engine is running rich or lean. This could be due to a faulty fuel injector, high or low fuel pressure, MAF sensor, engine coolant temperature sensor, or almost any faulty engine sensor. The air-fuel mixture needs to get addressed and then see if the P0420 code comes back.

Read More: Car Dashboard Symbols and Warning Lights

What repairs can fix the P0420 Code?

  • Replace the muffler or repair leaks in the muffler.
  • Diagnose any repair any misfiring problems
  • Replace or repair fuel injector(s) that are leaking
  • Replace the exhaust manifold or repair leaks in the exhaust manifold
  • Repair or replace oxygen sensor connector(s)
  • Replace exhaust pipe or repair exhaust pipe leaks
  • Repair or replace damaged wiring to oxygen sensor(s)
  • Replace catalytic converter (most common)
  • Replace front or rear oxygen sensor
  • Replace engine coolant temperature sensor
  • Fix lean or rich fuel mixture
  • Diagnose and repair any other related trouble codes that have been stored by the Power Control Module (PCM)

How serious is the P0420 code?

Low – The P0420 code will in most cases not create any further problems with your car’s engine.

The only thing that could potentially happen is that the catalytic converter is so damaged that the catalytic converter’s internal parts come loose and block the exhaust flow, which is pretty unlikely to happen, though.

However, the P0420 trouble code could result in worse emissions from your car, which is bad for the environment, so you should therefore fix it as soon as possible.

What is the repair cost of code P0420? 

The repair cost of the P0420 code varies according to the vehicle model, labor cost, and the nature of the repair. If this code is triggered due to a faulty catalytic converter, your mechanic may cost from $490 to $1600 to fix it.  

The estimated cost of repairing the P0420 code is given below. The prices include parts and labor work at a workshop. The costs do not include diagnosis costs.

Catalytic Converter Replacement 590$ to 1600$
Rear Oxygen Sensor Replacement140$ to 320$
Front Oxygen Sensor Replacement140$ to 310$

FAQ Section

How do I clear a P0420 code?

You can clear the P0420 code temporarily with a scan tool or code reader. The problem is that the code will come back as soon as your car’s primary computer runs its system self-tests. To get the code to go off and stay off, you’ll need to fix the underlying problem that triggered the code in the first place, then clear the code with a scan tool or code reader.

Can I drive with a P0420 Code?

Yes, you may be able to drive your car with a P0420 code, but it is not advisable. Driving your car with the P0420 code will only damage the catalytic converter further. Plus, the problem can affect other parts of the car, such as the fuel line and exhaust systems. This is why it’s important to actively monitor your car’s catalytic converter to avoid other costly repairs. You should consult a qualified technician any time you suspect any catalytic converter damage.

What is the P0420 code on a Nissan?

The P0420 code is a generic OBD code with a standard definition from the Society of Automotive Engineers (SAE). SAE refers to the code as “Catalyst System Efficiency Below Threshold (Bank 1)”. But on many newer Nissan vehicles, when using an OEM-level scan tool, the code may come up as “Three-Way Catalyst Malfunction.”

In either scenario, the code indicates that your car’s onboard computer perceives a decline in the efficiency of the catalytic converter(s).

What is the P0420 code on a Chevrolet?

If you experience a p0420 code in your chevy, it is often due to an exhaust leak before the catalyst. It can also be caused by a faulty downstream oxygen sensor (rear) or a faulty catalytic converter.

Chevy engines are known to leak around the exhaust manifold, so you should definitely take a look there if you can find some strange blowing sounds when your engine is running.

How to clear the P0420 code?

To clear the P0420 code, you must use an OBD-ii scanner. Remember that clearing your P0420 code will probably only temporarily solve your problem. To fix it permanently, you need to fix the underlying issue.

Can a bad O2 sensor cause a P0420 code?

Yes. A typical catalyst monitor uses the signal from the downstream oxygen sensor to determine catalyst efficiency, and the signal from the upstream sensor is a reference point. As such, failure of either the upstream or downstream oxygen sensor can cause code P0420 to set. The most common cause of the code, however, is a failed catalytic converter.

What could damage the catalytic converter?

  • Misfires
  • Bad PCM
  • Oil consumption
  • Lean mixture
  • Exhaust leak
  • Rich mixture
  • Intake leak

What is the most common cause for a P0420 code?

A bad catalytic converter is the most common cause of a P0420, but it can be caused by anything from a faulty oxygen sensor to a rich or lean running condition or misfires.

Will code P0420 fail emissions?

A P0420 engine code will also lead to a failed emissions inspection, so it’s a good idea to fix the problem if your state requires regular smog checks.

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