How to tell if catalytic converter is clogged?

Updated March 23, 2017

Since 1975, federal law has required all new vehicles to be equipped with catalytic converters. The catalytic converter is a device that uses precious metals to filter and control engine exhaust, reducing the amount of pollutants escaping from the tailpipe to the air. Troubleshooting a catalytic converter is not difficult, since modern vehicles have specific computers monitoring their function. You can learn to tell if your catalytic converter is clogged, damaged or faulty.

Drive your vehicle to test performance and engine operations. A vehicle with a clogged catalytic converter will exhibit excessive exhaust back-pressure. Testing for this is not difficult. Listen to your engine when accelerating. If the engine sputters, hesitates or experiences unequal power delivery, you have a problem. It may not be the catalytic converter, but once you have symptoms you can narrow the possibilities down.

Hook your vehicle's diagnostic connection (usually underneath the steering wheel, near the pedals) to an OBD-II scanner. Scanners can be purchased for as little as £52 and will work on all vehicles 1996 and newer. If you cannot acquire a scanner, take the vehicle to your local auto parts store. Many auto parts stores will hook a scanner to your diagnostic connection free of charge; they'll even let you write down the trouble code.

Analyse the trouble code coming from your vehicle. A clogged catalytic converter will always result in an OBD-II trouble code because the catalytic converter's function is so important to meet emission standards that the diagnostic system has a separate monitor for catalytic converter efficiency. Look for a trouble code ranging anywhere between P0420 to P0439, which would indicate catalytic converter trouble.

Replace your catalytic converter if it is clogged or faulty for some other reason. A clogged catalytic converter could cause extreme engine stalling if exhaust back-pressure gets too high. This could leave you stranded on the side of the road, or even in an accident if your engine stalls in heavy traffic.


If you have a trouble code of any kind when you pull your diagnostic record, have your vehicle checked out. Trouble codes of any kind can signal other problems, some very serious. If your vehicle is a 1995 model or older, you will need to bring it to a professional to diagnose the system. Before the 1996 year model, vehicles used a diagnostic system called OBD-I, which cannot be diagnosed with a simple scanner.


Do not remove your catalytic converter and drive your vehicle. This violates the Clean Air Act and is a federal offence. All vehicles are required to have catalytic converters and drivers are required to replace them when they are faulty.

Things You'll Need

  • OBD-II Scanner


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About the Author

Leonardo R. Grabkowski has been writing professionally for more than four years. Grabkowski attended college in Oregon. He builds websites on the side and has a slight obsession with Drupal, Joomla and Wordpress.