P0420 – Meaning, Causes, Symptoms, & Fixes
Code P0420 Definition
Catalyst system efficiency below threshold (Bank 1).
What Does P0420 Mean?
The function of the catalytic converter is to break down harmful pollutants that are created by your vehicle during the combustion cycle and convert them into less harmful gases. Code P0420 indicates that the catalytic converter is not functioning efficiently, therefore increasing the output of harmful pollutants by the vehicle.
What Are the Symptoms of Code P0420?
- Check Engine Light is on
- Lack of power from the engine
- Decreased fuel economy
- Rotten egg or a sulfur smell
What Is the Cause of P0420?
- Faulty oxygen sensor
- Faulty air-fuel sensor
- Worn or internally failing catalytic converter (Most Common)
- A leak in the exhaust system
- Misfire (the root cause of converter failure)
- Rich or Lean air-fuel ratio (the root cause of converter failure)
- Leaded Gasoline (the root cause of converter failure)
How Serious Is Code P0420? – Moderate
While P0420 does not pose a danger to the driver, it can cause driveability concerns such as a lack of power or poor acceleration if left unrepaired. It can also cause severe damage to other components of your vehicle if not addressed in a timely manner. It is recommended that you address P0420 as soon as possible to keep the cost of the repair to a minimum.
Common Diagnosis Mistakes for Code P0420
Often times this code is thought to be a faulty O2 sensor or A/F Sensor; while this is a possibility, the most common problem is a faulty catalytic converter. Do not overlook other codes that are paired along with P0420. Codes like P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308 are misfire codes, and these codes require misfire diagnosis. If the catalytic converter is replaced without repairing the misfire, the new catalytic converter will go bad again. Also, make sure you don’t have codes P0174, P0171, P0172, or P0175 which can mean the engine is running rich or lean, which can burn out your catalytic converter. If these codes or any other codes are present, they should be addressed first.
Tools Needed to Diagnose Code P0420:
How to Diagnose Code P0420:
Difficulty of Diagnosis: 5 out of 5
- Scan your vehicle to verify P0420 is the only code present. If other codes are present, they must be addressed first.
- Examine the exhaust system for damage and leaks. Pay close attention to the exhaust manifold, gaskets, and exhaust pipes pre-catalytic converter. If any leaks are found, repair the leak, clear the code, and complete several drive cycles to verify that was the fix.
- With the vehicle running and at normal operating temperature, use the digital multimeter to check the voltage reading of the downstream O2 sensor. The downstream O2 sensor produces a relatively steady voltage reading of approximately 0.45V if the catalytic converter is functioning properly. If the voltage of the downstream O2 sensor is constantly jumping between 0.1V and 0.9V, the catalyst is worn and the catalytic converter needs to be replaced.
Estimated Cost of Repair
For error code P0420, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.
- Air fuel sensor or oxygen sensor $200-$300
- Catalytic converter $400-$2400
- A leak in exhaust $100-$200 (if welded to repair)
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