What causes catalytic converter failure and a P0420 code

What Causes Catalytic Converter Failure and How to Prevent It

A catalytic converter converts toxic gases from combustion, like carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) into less harmful substances such as carbon dioxide (CO2), nitrogen (N2), and water vapor (H2O). A catalytic converter is designed to last the life of the vehicle. There are no moving parts inside. However, catalytic converters can fail over time, leading to poor vehicle performance, increased emissions, and diagnostic trouble codes (DTCs), notably the P0420 code. This article explores the common causes of catalytic converter failure and the implications of a P0420 code.

Understanding the Catalytic Converter Failure and the P0420 Code

The P0420 code is one of the most common DTCs and indicates that the catalytic converter is not functioning efficiently. Specifically, it suggests that the catalyst system efficiency is below the threshold for Bank 1, meaning the converter is not reducing emissions as effectively as it should.

The engine control module (ECM) monitors the performance of the catalytic converter through two oxygen sensors—one located before (upstream) and one after (downstream) the converter. The upstream sensor measures the amount of oxygen in the exhaust gases before they enter the catalytic converter, while the downstream sensor measures the oxygen levels after the gases have passed through the converter. If the ECM detects that the oxygen levels before and after the converter are too similar, it triggers the P0420 code, indicating that the catalytic converter is not performing efficiently.

The Most Common Causes of Catalytic Converter Failure

1) Engine Misfires— A misfire occurs when the air-fuel mixture in the combustion chamber does not ignite or burn properly, leading to unburned fuel

Overheated catalytic converter viewed from the outside

entering the exhaust system. It’s not uncommon for some excess fuel to enter the converter as the ECM adjusts the air/fuel mixture. However, when there are misfire problems, a lot of unburned fuel enters the catalytic converter. Large amounts of unburned fuel can cause the converter to overheat and melt the ceramic structure, which is the core of the converter. If not fixed quickly, persistent engine misfires can lead to severe catalytic converter damage over time.

2) Fuel Mixture Issues— An overly rich fuel mixture, where there is too much fuel and not enough air, can also damage the catalytic converter. This condition can occur due to faulty fuel injectors, a malfunctioning mass airflow sensor (MAF), or a failing oxygen sensor. When the fuel mixture is too rich, excessive hydrocarbons enter the exhaust system, leading to catalytic converter overheating and eventual failure.

3) Contamination Due to Internal Oil or Coolant Leaks— Oil or coolant leaks into the combustion chamber can be particularly harmful to the catalytic converter. These leaks can occur due to worn piston rings, valve seals, or a blown head gasket. When oil or coolant burns in the exhaust system, it creates a thick residue that coats the catalyst material inside the converter, reducing its efficiency and leading to eventual failure.

4) Contamination Due to Fuel or Additives—Contaminated fuel containing high levels of lead, sulfur, or driver-added anti-wear additives containing ZDDP or other impurities can also cause catalytic converter failure. These contaminants can coat the catalyst material, preventing it from effectively reducing emissions. Although leaded gasoline is no longer commonly used, other contaminants can still enter the fuel system and damage the catalytic converter.

5) Physical Damage— Physical damage to the catalytic converter, such as from road debris or accidental impact, can crack or break the ceramic substrate inside the converter. This damage can reduce the converter’s efficiency and lead to the P0420 code. Additionally, improper installation or handling of the catalytic converter during repairs can also cause physical damage.

A bad oxygen is almost never the cause of a catalytic converter failure or a  P042o code

I know that many online sites tell you to fix a P0420 with a new oxygen sensor. In rare cases, that might fix the problem. But for the vast majority, replacing the oxygen sensor is really wishful thinking and a waste of money.

The rare cases when an oxygen sensor could cause a P0420 or P0430

On many pre-1996 vehicles, the upstream oxygen sensor uses a “narrow band” type sensor. These sensors compare the amount of oxygen in the exhaust to the amount of oxygen in the outside air. It obtains and ambient sample through one of the wires (between the copper and the insulation). If the engine has dripped oil onto the electrical connector or dirt/debris has coated the wires, the sensor will give inaccurate data to the ECM.

So it’s possible a bad narrow band sensor can cause a P0420 or P0430, but it’s rare. Checking the condition of the O2 sensor connector and wires should be part of every diagnostic when encountering a cat converter code.

What can you do to diagnose the cause of a catalytic converter failure and a P0420 code?

• Check for an exhaust system leak— An exhaust leak upstream of the catalytic converter can cause a false P0420 or P0430 code. Here’s how to conduct an exhaust leak test.

• Check long-term fuel trims— If the ECM is adding a lot of fuel 25%, that indicates a problem. Fix the underlying problem and bring long term fuel trims back to an acceptable level before replacing the catalytic converter.

• Check the catalytic converter for signs of exterior damage— Check for dents or signs of overheating

• Check exhaust backpressure— Connect a vacuum gauge to the intake manifold. Note the reading at idle. Then, raise RPMs to around 3,000. As you open the throttle, it’s normal to see a quick drop in vacuum, but it should then rise and level off. If the vacuum starts to drop off as you hold the RPMS steady, that’s the sign of a blocked cat converter. In other words, the passages through the ceramic honeycomb either melted or become clogged, or there’s an exhaust restriction in the muffler or resonator.

The temperature input versus output temp test is no longer considered a valid test

In the early days of catalytic converters, the rule was that the input versus output temperatures must be at least 100°F apart. But better fuel injection and fuel metering and more efficient combustion have resulted in cleaner-burning engines. So, the difference in input versus output temperatures in a late-model vehicle can often be as little as 50°F at 2,500 RPM.  That’s not enough of a difference to rule the catalytic converter, good or bad. In fact, at idle, the converter can cool down to the point where the difference may actually be 0°F. If you replace a catalytic converter simply because the outlet temp matches the inlet, you may be replacing a perfectly good converter.

Tips to avoid catalytic converter contamination:

NEVER use any type of RTV on exhaust flanges. If a gasket won’t seal it, don’t reach for any type of sealer. You’ll just kill the cat converter. Have an exhaust shop weld on a new flange so it will seal with a gasket.

NEVER use Teflon tape on an O2 sensor. High-temperature anti-seize is the ONLY sealant that’s approved for O2 sensors.

NEVER add high-temperature silicone RTV to O2 sensor threads. High-temperature anti-seize is the ONLY sealant approved for O2 sensors.

©. 2019 Rick Muscoplat