How to tell if catalytic converter is clogged?

Written by leonardo r. grabkowski
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How to tell if catalytic converter is clogged?
A clogged catayltic converter can reduce engine performance and efficiency. (

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.

Skill level:
Moderately Easy

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Things you need

  • OBD-II Scanner

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  1. 1

    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.

  2. 2

    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.

  3. 3

    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.

  4. 4

    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.

Tips and warnings

  • 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.

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