Catalytic Converter: problems, replacement options

October 09, 2022
The catalytic converter or "cat" is a key component of the vehicle emission control system. It reduces harmful emissions coming out of exhaust. The catalytic converter is built inside the exhaust manifold or within the exhaust pipe close to the exhaust manifold.
Catalytic ConverterCatalytic converter. Larger image.

Gasoline-powered cars have "Three-Way" Catalytic converters or TWC that reduce emissions of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Read the paragraph below How do three-way catalytic converters work?

Some vehicles have one catalytic converter or TWC for each cylinder bank. Many modern cars and trucks have more than one catalytic converter, so if you have a problem with your catalytic converter, it's important to know which one.

The primary or front three-way catalytic converter is placed close to or within the exhaust manifold.
Catalytic ConverterCatalytic converter exhaust emission control. Larger image.
If it's an engine with two cylinder banks (e.g., V6 or V8) it will have two front catalytic converters. In most cars, only the front catalytic converter is monitored by the sensors. The Air/Fuel ratio sensor (A/F ratio sensor) is installed before the front catalytic converter and the rear oxygen sensor HO2S is installed downstream from the front catalytic converter. There may be another secondary or downstream catalytic converter placed further in the exhaust pipe. It could be called 'underbody catalyst'.
In some cars, the front catalytic converter is designated as a "warm-up" three-way catalytic converter, or WU-TWC, while the downstream converter is called TWC.

If you want to know about the catalytic converter setup in your vehicle, check the Vehicle Emission Control Information sticker.
Vehicle emission control information stickerVehicle emission control information sticker. Larger image.
Typically it's located under the hood or in the engine compartment. For example, this sticker from the 2009 Honda Accord, located on the inside of the hood, says that it has WU-TWC, TWC as well as air/fuel ratio or A/F sensor and rear O2 sensor, or HO2S.

If it's a modern vehicle with a V6 or V6 engine with two cylinder banks, there are at least two catalytic converters, one for each bank. Both front catalytic converters in a V6 or V8 engine are also monitored with front A/F and rear O2 sensors. Cylinder banks are designated Bank1 and Bank 2; Bank1 is the one that contains the cylinder number 1.
The catalytic converter is covered by the 8-year 80,000-mile Emission Warranty.

Catalytic Converter Problems

1. Check Engine light staying on, with further diagnosis revealing trouble codes related to the catalytic converter: P0420, P0430 or P0421 and P0431. All these codes point to the efficiency of the catalytic converter falling below the threshold. We wrote a separate article that explains these codes and provides the information on testing: P0420 - Catalyst Efficiency Below Threshold.

2. Clogging up of the catalytic converter. The photo on the left shows a clogged up catalytic converter from one of the cars that we had to repair.
Clooged up versus clean catalytic converterClogged up versus clean catalytic converter. Larger image
Symptoms of a clogged up catalytic converter include a lack of power and the engine running hot. The car may run well at idle, but bog down when accelerated. We have also seen EGR valves burned out as a result of a clogged up converter.

Mechanics test the intake vacuum and measure exhaust back pressure through the air/fuel ratio sensor opening to confirm a clogged-up catalytic converter.
If the intake vacuum drops when RPMs increase or if the backpressure measured at the air/fuel ratio sensor opening is more than 2.5 psi at 2,000 RPMs, the catalytic converter is likely plugged up.
Many shops have borescope cameras that mechanics use to check for blockage visually.
There are many reasons that can cause the catalytic converter to clog up, including bad gasoline, increased oil consumption of the engine and coolant leaks from the head gasket, misfiring. Often this happens to cars that are driven only for short trips.

Clooged up versus clean catalytic converterMelted catalytic converter inside.
Larger image
3. Burned through, melted or disintegrated catalytic converter. Symptoms of this problem are a rattling noise coming from the catalytic converter and the Check Engine light with the codes mentioned above. You may notice the smell of unburned fuel from the exhaust, like from old cars from the 50s. As we were preparing this article, we worked on a car with the catalytic converter code P0421. We cut the catalytic converter open to show you how it looks. It's partially melted and disintegrated as you can see. Read how to test the catalytic converter with a scan tool or an infrared thermometer in this article: Code P0420 Catalyst Efficiency Below Threshold (Bank 1)

4. If the catalytic converter shows discoloration and other signs of overheating or physical damage such as dents or cracks that cannot be repaired, or if the catalytic converter is built into the exhaust manifold that is cracked or has a broken runner, it will also have to be replaced.

Stolen catalytic converterThe catalytic converter has been stolen from this vehicle. Larger image
5. Another problem is a sign of our times. It's when you start your car only to find out that it sounds like a sports car. It probably means that the catalytic converter has been stolen. Unfortunately, we see a growing trend of catalytic converter thefts. We know of one case when the catalytic converter has been stolen twice from the same car. The second time it happened on the next day after it had been repaired.

In either of these cases, the catalytic converter will need to be replaced. If it has been cut off, some other parts might need to be replaced too, depending where they cut it. If the air/fuel ratio sensor or oxygen sensor in the old converter is seized and cannot be removed without damaging it, or if it has been stolen together with the cat, it will need to be replaced too.
It's also important to find out what caused the catalytic converter to fail. If it's because the engine consumes a lot of oil, or due to some other engine defect, the new catalytic converter will not last long if the root cause is not repaired.

Catalytic Converter Replacement Options

Catalytic ConverterFront catalytic converter (TWC). GMC Sierra.
This is not the cheapest of the repairs. Original (OEM) catalytic converters are expensive so consider all your options. Some states (e.g., California, New York, Maine, Colorado) have stricter standards and require the replacement catalytic converters to be CARB-compliant (California Air Resource Board). Check your local laws first.

1. The first option is to visit the dealer and have them replace the catalytic converter with a new OEM part. This option is good if you want to use the original part and don't mind paying top dollars for it, or if your insurance covers it. Your dealer will not have the part in stock, but they should be able to get it for the next day or within a few days if the part is available.
Front catalytic converter assemblyOEM front catalytic converter assembly.
Of course, we did some research and found that for certain models that are common targets for catalytic converter theft, parts are not available or on back-order.
In an average car with a 4-cylinder engine, the catalytic converter replacement will cost 1.5-2.0 hours of labor plus the part and hardware plus a diagnostic charge if it was needed. Some OEM catalytic converters are priced over $1,000. Some CARB-certified catalytic converters cost more than double of that.

2. The second option is to use a direct-fit aftermarket catalytic converter. Check Amazon or eBay indicating your car's year, make, and model or search Google for online prices. You can find a much cheaper aftermarket cat that fits your car. You can have it installed at any shop, but it's important to inquire about the warranty; we know cases when after installing the aftermarket catalytic converter the Check Engine light with the catalytic-related fault code came back. For example, some Chrysler vehicles are known to set the code when aftermarket catalytic converters or air/fuel ratio sensors are used. The labor charge will be the same, 1.5-2.0 hours for an average 4-cylinder car, but the part will be cheaper than the OEM.

3. The third option is a universal catalytic converter which is much cheaper, although it will not fit all cars. For example, it will not fit in cars that have the catalytic converter built inside the exhaust manifold as one unit. In some cars and trucks, however, a universal converter can be welded fairly easily in or even attached with clamps. Check with the local muffler shop as they may be able to fabricate the part on the spot using a universal converter. We recently got this option done at our local muffler shop for the Subaru Forester we had and it worked well.

4. The fourth option, is to buy a used part. This option is often considered if the catalytic converter is built inside the exhaust manifold. A used catalytic converter might be harder to find and the condition of a used part is questionable, but we have seen cases where a used part was the most affordable option and it worked. Your repair shop can check with local auto recycling facilities (wreckers) to source the part.

How do Three-Way Catalytic Converters Work?

Catalytic Converter ceramic monolithCatalytic converter inside. Ford display. Larger image.
Modern gasoline-powered cars have "three-way" catalytic converters, or TWC. The three-way catalytic converter is encased in a metal housing within the exhaust. It has a ceramic honeycomb-like structure (monolith) that typically has square cross section channels, see the photos.

The monolith is coated with catalyst elements, such as Platinum (Pt), Rhodium (Rh), Palladium (Pd) and Ruthenium (Ru). A catalyst is a substance that enables a chemical reaction but is not consumed in it. As exhaust gases pass through a sufficiently warmed-up catalytic converter, chemical reactions take place. The job of a three-way catalytic converter is:

1. Burn off unburned fuel and oil vapors (hydrocarbons or HC), converting them into carbon dioxide (CO2) and water (H2O). The simplified formula is:
a CxHy + b O2 → c CO2 + d H2O
2. Convert carbon monoxide (CO) to carbon dioxide: (CO2):
2 CO + O2 → 2 CO2
3. Reduce emissions of nitrogen oxides (NOx), converting them into nitrogen (N2) and oxygen or carbon dioxide, for example, one of the ways is:
2 NO + 2 CO → N2 + 2 CO2
Catalytic Converter honeycombCatalytic converter honeycomb. Larger image.

The three-way catalytic converter is more effective when the engine is running at stoichiometric air-fuel ratio which is about 14.7:1 for gasoline engines. It means 14.7 parts of air to 1 part of gasoline fuel. To maintain the stoichiometric air-fuel ratio, the engine computer or PCM uses the signal from the air/fuel ratio sensor (front O2 sensor) that is installed before the primary catalytic converter. The rear O2 sensor monitors the efficiency of the catalytic converter. See how the sensor signals look on the scan tool in the article: Code P0420 Catalyst Efficiency Below Threshold (Bank 1)