A catalytic converter reduces vehicle emissions and pollution. It is filled with a chemical catalyst, usually a platinum and palladium mixture, that helps to convert the engine’s emissions into non-harmful gasses. But sometimes, the catalytic converter can get clogged up, hurting your engine’s performance and depriving you of the pep that you previously had. Hence, it is important to know how to tell if a catalytic converter is clogged or not.

Additionally, a failing converter can cause your car to have poor gas mileage, so this can affect your pocket book on a dat-to-day basis, as well.

A clogged catalytic converter can be a common problem in really high-mileage vehicles. This is usually around the 250-300 mile range, although it may come sooner.

On the whole, catalytic converters rarely clog, and even if they do, it’s mostly because there is something wrong with the vehicle that causes the clog, like engine misfires, poor quality fuel, etc. All of these problems put extra strain on the catalytic converter and cause carbon build-up, which gradually clogs it if left untreated.

Usually, a clogged catalytic convert will be accompanied by a “Check Engine (P0420)” light. Otherwise, a catalytic converter can demonstrate a clog via:

• Poor Acceleration
• Capped Speed
• Engine RPM instability

Catalytic converters are pretty expensive, and you wouldn’t want to guess. So If you’re unsure, check the full list of symptoms and signs below indicating a clogged catalytic converter: 

1. High Exhaust Backpressure / O2 Voltage

The best way to know if your catalytic converter is clogged is to use either an OBD2 scanner or an exhaust backpressure gauge. With an OBD2 scanner, your reading should be around 0.5-0.7V and shouldn’t fluctuate much. While with an exhaust backpressure gauge, the normal backpressure should be around 1.5 PSI.

Exhaust Backpressure Gauge Reading

If you’re using a catalytic converter backpressure pressure gauge, it is very easy to see whether it’s clogged or not, the chart below shows the severity level based on PSI.

1-1.5 PSI	Perfect
2.5- PSI	Ok
3+ PSI	Clogged

How To Measure The Exhaust Backpressure

Credits to Scotty Kilmer for the images and the instruction!

To measure the exhaust backpressure and determine if it clogged or not, do the following procedure:

• Get an Exhaust backpressure gauge
• Locate the O2 sensor in front of the catalytic converter

• Unbolt the O2 sensor
• Remove it from the hole

• On older cars, you should use a rubber adapter that pushes in the hole
• On newer cars where the oxygen sensor has a thread that screws in, you need to use an adapter, which should come with the exhaust backpressure gauge kit
• The threaded part of the adapter screws in the hole, while the other part is for the tester
• Push/Screw it in the hole and start the car

• Use the chart above to understand what readings are fine, and which are not.

Note: If you do have high back pressure before the catalytic converter, you’ll also need to check it behind the catalytic converter. If it still has high pressure behind the catalytic converter, then your muffler may be clogged. This happens if your catalytic converter is so clogged that pieces of debris break off and get stuck in the muffler. In that case, you’ll need to clean/replace both the catalytic converter and the muffler.

OBD2 Scanner O2 Sensor Reading:

With an OBD2 scanner, it’s a little more complicated to understand the O2 sensor readings and what they mean. As for the simple answer, if the O2 exhaust sensor voltage fluctuates a lot, your catalytic converter is clogged. Although, do keep in mind that it is also possible for the muffler to be clogged if your catalytic converter got so bad that parts of the debris flew out into the muffler.

5% Fluctuation	Perfect
10% Fluctuation	Good
30%+ Fluctuation	Clogged

How To Check The O2 Sensor Readings

To check what voltage your O2 sensor is, and determine if it is clogged or not, do the following procedure:

1. Take a 10-15 minute drive.
2. Use an OBD2 scanner capable of live data reading
3. Find the responsible O2 sensor on the OBD2
4. Your O2 sensor voltage should be around 0.5-0.7V.
5. Make sure to check both O2 banks.
6. Pick any number on the reading, let’s say 0.6. If the catalytic converter is perfectly functional it would have about 5% fluctuation, which would keep it in the range of 0.57V-0.63V, as 5% of 0.6 would be 0.03. Use the chart above to guide you.

2. Engine Problems

Engine problems could be in the form of poor acceleration, difficulty starting the engine, rpm instability, capped speed, etc. This is because the catalytic converter doesn’t push out the exhaust gasses fast enough, “choking” the engine.

Often the temperature gauge will run a little bit hotter because the exhaust gasses are staying in the engine for too long, making it run hotter. Or the car will only go up to a certain speed and no further.

But of course, many other car problems can cause all those symptoms, so it’s important to check the backpressure or the OBD2 voltage first.

3. Check Engine Light

The check engine light often appears if your catalytic converter is clogged, although since the O2 sensor reports slower (because it measures efficiency over a longer period of time than other sensors), you might get a “check engine” light for something else like engine misfires, before you get a check engine light for a clogged catalytic converter. If you connect a scanner and receive “P0420” which means “Catalyst System Efficiency Below Threshold” you can assume your catalytic converter is to blame, although there can be different reasons such as:

• Faulty catalytic converter
• Faulty front/rear oxygen sensor
• Damaged/Leaking muffler
• Damaged/Leaking exhaust manifold
• Damaged/Leaking exhaust pipe
• Engine Misfire
• High fuel pressure
• Oil contamination in the catalytic converter

Related: What Causes The Check Engine Light To Come On?

4. Hotter Than Usual

Another way to tell if your catalytic converter is clogged is by the use of the infrared thermometer. Since clogged catalytic converters tend to run hotter, you can check the temperature of the exhaust pipe before the catalytic converter and after. The normal operating temperature can range from 1,200 to ~1,700 degrees F. But as the number of pollutants in the exhaust chain go up, so does the catalytic converter’s operating temperature.

Note: Some cars can have multiple catalytic converters, but generally they go bad at the same time.

To check, do the following procedure:

• Check the temperature before the catalytic converter using the exhaust pipe, since you can’t measure the exhaust gasses directly

• Check the temperature after the catalytic converter.

• By the big jump in temperature, it can be assumed that this car has a clogged catalytic converter

5. Failed Emission Test

States like Georgia, Massachusetts, New Hampshire, New York, Vermont, and others, require annual safety inspections that include an emissions test. So obviously if your catalytic converter is clogged or damaged, you won’t pass the emissions test.