How To Unclog a Catalytic Converter

You can unclog your car’s catalytic converter with a catalytic converter cleaner or by removing the part to pressure wash and degrease it.
Written by Pat Roache
Edited by R.E. Fulton
You can unclog your car’s catalytic converter with a catalytic converter cleaner or by removing the part to pressure wash and degrease it before reinstalling it. 
  • A fuel additive won’t require you to touch your catalytic converter, but it’s not as effective as removing the part and pressure washing it.
  • Common signs of a clogged catalytic converter include decreased fuel efficiency, poor engine performance, and the smell of rotten eggs.
  • Unclogging your catalytic converter is an easy and cheap alternative to replacing it, but it’s not always possible.

How to unclog a catalytic converter without removing it

Catalytic converter cleaner is a fuel additive that you can pour into your fuel tank to clean your catalytic converter without removing it.
 Here are a few examples of commercial catalytic converter cleaners:
Whatever brand you choose, read the instructions carefully before using the product. Here’s what you can generally expect:
  • Your car needs to be at about a quarter tank of gas, but different products may have different fuel level requirements.
  • You’ll have to drive your car around until it’s nearly empty to let the catalytic converter cleaner circulate through your exhaust system.
  • You’ll be able to tell that the catalytic converter cleaner has worked if your exhaust smoke is lighter in color, not as smelly, or your car accelerates more smoothly.
Keep in mind: Catalytic converter cleaners are not likely to be as effective as removing your catalytic converter to pressure wash it. Typically, a catalytic converter cleaner will not work at all if your catalytic converter is too dirty. 

How to unclog a catalytic converter by removing it

You’ll need the following equipment to unclog a catalytic converter by removing it:
  • Automotive degreaser 
  • Penetrating oil 
  • Jack and jack stand 
  • Impact wrench 
  • Pressure washer 
First, you’ll need to remove the catalytic converter:
  • Make sure your exhaust system has had time to properly cool down before you begin.
  • Raise your vehicle on jack stands.
  • Locate and remove the oxygen sensor. 
  • Apply the penetrating oil around the bolts holding the catalytic converter in place to loosen them. 
  • Undo the bolts with the impact wrench and remove the catalytic converter. 
Before you move on: Give your removed catalytic converter a shake to check for any rattling sounds. These could indicate that your catalytic converter is damaged beyond repair. 
Next, use your pressure washer on a low-pressure setting to wash out your catalytic converter. Make sure to properly dispose of any hazardous waste that falls out.
Pro tip: Soak your catalytic converter in automotive degreaser to loosen up grime and debris before tackling it with your pressure washer.
Once you’re done, allow the catalytic converter to completely dry before reinstalling it with the oxygen sensor.
MORE: How to put a car on jack stands

Will unclogging your catalytic converter work?

Unclogging a malfunctioning cat is an easy way to fix your catalytic converter without replacing it, but it’s not always possible. 
Here are some signs that your catalytic converter is beyond repair:
  • A rattling noise coming from the center of your vehicle, or wherever your catalytic converter is located
  • Visible physical damage or discoloration on the catalytic converter
  • The rotten egg smell in your car
When in doubt:
Take your car to a mechanic
to figure out whether or not your catalytic converter needs to be unclogged or replaced.

How to diagnose a clogged catalytic converter

You may have a clogged catalytic converter if you notice any of the following symptoms:
  • Engine issues: Your engine won’t start, it sputters and stalls out, or you notice an
    engine knock
    at slower speeds. 
  • Fuel efficiency issues: Your
    car gets worse gas mileage
    because your engine has to work harder and consume more fuel due to a block in the catalytic converter. 
  • Failed vehicle emission test: Perhaps the most telling sign. Your vehicle fails
    emissions testing
    and is kicking out more harmful substances than regulatory standards. 
  • Foul smell/rotten egg smell: A rotten egg smell in your car points to sulfur—one of the harmful gasses that the catalytic converter is supposed to convert into a safer emission.
  • Your
    check engine light
    comes on: Your car’s diagnostic systems alert you to an issue with one of your components. Check on the catalytic converter. 
Perform one of the following diagnostic tests if you suspect a clogged catalytic converter.

Vacuum test

  • Grab a vacuum gauge and a driving partner. 
  • Hook up the vacuum gauge to the direct intake manifold or another direct vacuum line while the car is off. 
  • Turn your car on, allowing it to warm up to its normal operating temperature. The vacuum gauge should read 18 to 22 inches of Mercury.
  • One person should drive the car at a consistent 2500 to 3000 RPM while the other monitors the vacuum gauge from the passenger seat.
  • Pay attention to whether or not the gauge quickly returns to its starting level between 18 and 22 inches of Mercury after any decreases.
You may have a clogged catalytic converter if the vacuum gauge doesn’t return to its former level after any decreases within a few seconds.

Temperature test

  • Allow your car to idle on level ground until it reaches its normal operating temperature.
  • Use an infrared or kitchen thermometer to take the temperature of the inlet pipe connected to the front of your catalytic converter.
  • Take the temperature of the outlet pipe connected to the rear of your catalytic converter.
The outlet pipe temperature should be 50 to 100°F hotter than the inlet pipe. You may have a clogged catalytic converter if the two temperatures are reading similarly or if the inlet temperature is higher.

Pressure test

  • Replace the front oxygen sensor on your vehicle with a back pressure gauge while your vehicle is off.
  • Turn your vehicle on and check the back pressure gauge.
You may have a clogged catalytic converter if the back pressure gauge has a reading higher than 3 PSI.

How to prevent a clogged catalytic converter

Following a strict
car maintenance schedule
with a regular oil change routine is crucial to keeping a healthy catalytic converter. 
Pro tip: Switch to a high-mileage oil once your car has passed 75,000 miles to make your oil changes as effective as possible.
MORE: How much should an oil change cost?
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