P0420 Catalyst System Efficiency Below Threshold - Diagnostic Trouble Code (P0420)

Causes, Symptoms, Severity and Cost

Technical Description:

The catalytic converter becomes less efficient in lowering pollution.

What P0420 really means?

The purpose is to lower the three main causes of pollution produced through gasoline combustion - carbon monoxide, nitrous oxide, and hydrocarbons (unburnt fuel). The toxic exhaust passes through a series of heated screens or honeycombs, which are coated with precious metals. These heated metals act as a catalyst that precipitates a chemical reaction, turning hydrocarbons into carbon dioxide and water, carbon monoxide into carbon dioxide, and nitrous oxide back into nitrogen and oxygen. Two oxygen sensors set into the exhaust before and after the catalytic converter provide the information to the computer about how well the catalytic converter is performing.

What are the causes of a P0420 code?

When the check engine light turns on for this code, we can really swear under our breathe, because more often than not it means a big repair bill. And the fault is mostly our own, too. Most catalytic converters fail because they are overworked. The primary reason is too much-unburned fuel is in the exhaust either from a poorly running engine or too low of a fuel octane rating for the car. Additionally, unresolved coolant or oil leaks that contaminate the combustion chamber also significantly shorten the life of a converter.

What is the severity of a P0420 code?

Minor. Look to make an appointment when it is convenient to lose the car for a day.

What repair is typically needed to resolve a P0420 code?

Catalytic converter replacement: On most cars, unless the engine is running poorly when this code sets, the problem is emission rather than performance related. The code must be addressed before any state emission test, but your car is not in danger of breaking down. Not much diagnosis is needed beyond ensuring that the oxygen sensors are working correctly (remember they police converter efficiency).

Either way, you look at it, the replacement of catalytic converter is expensive. In some cases this repair will reach into the thousands of dollars range, and unquestionably, the type of car you drive will determine the price.

Short term solution: for the domestic market, prices will generally be lower, and options will be available for aftermarket or non-factory converters. These non-factory converters will work nearly as well as the original factory converter and can bring the price of the job down into the $500 to $800 range. Generally, they do not have the same amount of precious metal as the original, so they are not as efficient, but they will keep the check engine light off. For foreign applications, the efficiency threshold is much tighter. Less expensive aftermarket converters are available, but the check engine light returns within a short interval.

Long-term solution: invariably the factory converter must be used for the permanent repair. Here, the price of the job will rise into the $1200 to $1500 range, possibly higher on exclusive models.

Keep in mind, pricing will vary by location and your vehicle make and model. If you don't already, save time and money by using Openbay to compare pricing and book an appointment with a service center in your area.

*P-Code meaning *

P = Powertrain,

0 = Generic,

4 = Emission Control,

20 = related to a particular issue

Service article written by an ASE Master Technician

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