P0420 Code: Meaning, Causes, and Symptoms | OBD2 Codes

P0420 – Catalyst System Efficiency Below Threshold Bank 1

P0420: What does it mean?

Code P0420 means that the catalyst system efficiency is below the threshold.

The Catalyst Converter is designed to break down harmful pollutants created by the engine during the combustion cycle. The precious metal coated in the heated screens or honeycombs acts as a catalyst that precipitates chemical reactions that turn toxic exhaust from the engine into less pollutive gases. 

Two Oxygen sensors are placed before and after the catalyst converter to test its efficiency. If the catalyst converter is performing well, there should be a difference between the data from two O2 sensors. The code P0420 is set when the computer(ECU) found out that the O2 level of the downstream sensor is not steady or the gap between two sensors is not as intended(which means the catalyst converter is not working as efficiently).

Severity: Moderate

As you may not feel any drivability issue, it’s a minor issue. However, it should be addressed as it might develop into a big problem (especially as the catalyst converter is expensive to replace). And you cannot pass the emission test with a code P0420 present.

Difficulty of diagnosis or repair: 5 out of 5

P0420 Possible Symptoms

P0420 is not always noticeable except the engine light/service engine soon warning light. Sometimes, you can feel the lack of power from the engine and decreased fuel economy. 

  • Engine Light ON (or Service Engine Soon Warning Light)
  • Lack of power from the engine
  • Decreased fuel economy
  • Rotten egg or a sulfur smell

P0420: Possible Causes

The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (

There are many possible causes for the code P0420, including:

  • Faulty Three-way Catalyst Converter Bank 1
  • Exhaust Tube
  • Intake Air Leaks
  • Faulty Oxygen (O2) Sensor
  • Faulty Fuel injector(s)
  • Leaking Fuel Injector (s)
  • Faulty Spark plugs
  • Improper Ignition Timing
  • Faulty Engine Control Module (ECM)
  • Dirty Air Filter

Possible Checks/Fixes you can do yourself:

When you see a DTC Code P0420, there’re several checks you can do:

  1. Check the code from your OBD2 Scanner / Car Code Scanner and verify that it’s the only code present. If other codes are on the list, they should be checked first. Sometimes, when you see O2 sensor related codes, fixing those might also fix the code P0420.
  2. If the code P0420 is the only code present, visually inspect any exhaust leaks before the rear O2 sensor. An exhaust leak may mislead the O2 sensor and trigger the code. If you find any broken, cracked or disconnected hoses, replace them and fix the code. 
  3. If a replaced hose does not solve the problem, use a digital multimeter to check the voltage reading of the rear O2 sensor. If the voltage reading is relatively steady around 0.45V, then the catalyst converter is functioning properly. If not, the O2 sensor may be replaced.
  4. If you find a malfunctioning catalyst converter, or if the code comes back after the fixed hoses, you may need to take your vehicle to a mechanic. They will inspect the vehicle with pro tools and fix the problem.

How much is the cost to clear P0420?

For car code P0420, several repairs may apply, and the cost estimations are below:

  • Replace the oxygen sensor $200-$300
  • Replace the catalytic converter $400-$2400
  • Repair a leak of exhaust $100-$200