Error P0420 – how to fix?

Code P0420 Definition

Catalyst System Efficiency Below Threshold (Bank 1).

P0420 Catalyst system efficiency below bank 1 threshold

What does P0420 mean?

The catalytic converter’s job is to break down the harmful pollutants that are produced by your vehicle during the combustion cycle and convert them into less harmful gases. The code P0420 indicates that the catalytic converter is not working efficiently, thereby increasing the emission of harmful pollutants by the vehicle.

What are the symptoms of the P0420 code?

  • The Check Engine light comes on
  • No power from the engine
  • Reduced fuel economy
  • Rotten egg or sulfur smell

What is the cause of the code P0420?

  • Defective oxygen sensor
  • Defective air-fuel sensor
  • Worn or internally inoperative catalytic converter (most common)
  • Leak in the exhaust system
  • Misfire (root cause of converter failure)
  • Rich or lean fuel-air ratio (root cause of converter failure)
  • Lead gasoline (primary cause of converter failure)

How serious is the P0420 code? – Moderate

While P0420 does not pose a risk to the driver, it can cause driving problems such as lack of power or poor acceleration, if left untreated. It can also cause serious damage to other vehicle components if not repaired in a timely manner. It is recommended that you deal with P0420 as soon as possible to keep repair costs to a minimum.

Common misdiagnosis for the P0420 code

Often this code is considered a faulty O2 sensor or A / F sensor; while possible, the most common problem is a faulty catalytic converter. Do not overlook other codes that are paired with P0420. Codes such as P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, ​​and P0308 are misfire codes, and these codes require a misfire diagnosis. If the catalytic converter is replaced without repairing the misfire, the new catalytic converter will fail again. Also, make sure you don’t have any P0174, P0171, P0172, or P0175 codes which may mean your engine is running rich or lean which could burn your catalytic converter up. If these codes or any other codes are present, they should be addressed first.

Tools needed to diagnose the P0420 code:

How to diagnose the P0420 code:

Difficulty in diagnosis: 5 out of 5

  • Scan your vehicle to see if P0420 is the only current code. If other codes are present, deal with them first.
    Inspect the exhaust system for damage and leaks. Pay particular attention to the exhaust manifold, gaskets and exhaust pipes in front of the catalytic converter. If a leak is found, repair it, clear the code, and perform a few driving cycles to verify that it was a fix.
  • With the vehicle running and at normal operating temperature, use a DMM to check the O2 sensor voltage reading. The O2 sensor produces a relatively constant voltage reading of about 0.45V if the catalytic converter is functioning properly. If the O2 sensor voltage continues to jump between 0.1V and 0.9V, the catalytic converter is worn out and must be replaced.
    Estimated repair cost
  • For error code P0420, one or more of the following fixes may be required to resolve the underlying problem. For each possible repair, the estimated repair cost includes the cost of the relevant parts and the labor required to complete the repair.

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