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Symptoms of a Bad Catalytic Converter


A catalytic converter is an exhaust emissions control device used in both gasoline and diesel engines. Often referred to as a "cat," its job is to convert a vehicle's exhaust emissions into a less harmful gas before the exhaust exits the tailpipe.

A catalytic converter is a metal canister installed in the exhaust system near the engine. It contains precious metals such as platinum, palladium and rhodium. When raw exhaust flows through it, multiple chemical reactions occur. The final product is an exhaust that's free of nitrogen oxides, carbon monoxide and hydrocarbons, three very harmful gases not only to the environment, but also to humans.

Catalytic converter

Signs of a bad catalytic converter

  • An illuminated check engine light. Your vehicle's engine computer receives feedback from sensors placed in the exhaust system called oxygen sensors. One sensor gets placed before the cat, and is called the "upstream" oxygen sensor. The other is placed after the cat, and is called the "downstream" oxygen sensor.

    Since the job of the catalytic converter is to filter out harmful gases from the engine's exhaust, the reading of the two sensors should be different. If they are the same, then that indicates the catalytic converter is not doing its job. In this situation your check engine light should come on and will set a code for an inefficient catalytic converter.
  • A failed vehicle emissions test. Some areas in the U.S. require a diagnostic check of the engine's computer to pass the emissions test. If a car has a bad catalytic converter, a trouble code will be stored in the computer, and the vehicle will not pass the test.
  • An unusual exhaust smell. Gasoline contains sulfur that turns into hydrogen sulfide during engine combustion. The catalytic converter changes this into sulfur dioxide, which is odorless. An inefficient or failing cat can leave more unburnt fuel in the exhaust which will smell like sulfur or rotten eggs.
  • A lack of engine power. In some cases, a catalytic converter will become plugged up and cause an exhaust restriction. This is normally caused by unburnt fuel igniting in the catalyst material and causing it to deteriorate. The exhaust restriction will severely impact the power output of the engine and require more throttle input to move the car. Eventually, the cat can completely block exhaust flow, and the engine will not run.
  • A rattling noise under the vehicle. The catalyst material is shaped like a honeycomb inside of its metal casing. A failing cat can break apart and cause a rattling noise while the engine is running. If it's not replaced, parts of the catalyst can break off and get stuck in the vehicle's muffler or resonator further down the exhaust stream.
Get it diagnosed by a professional

How to troubleshoot a catalytic converter

To determine if your catalytic converter has failed, a professional technician is needed. The tech will begin with a physical inspection and check to see if the catalyst material has broken apart, causing a rattling noise. In a situation of engine power loss, an exhaust system backpressure test will be performed. In some cases, a computer diagnostic can inform a tech of a poor performing cat through data relayed from sensors placed before and after the cat, called oxygen sensors.

Almost all catalytic converter failures are due to an issue with the engine. An engine that is not performing like normal can ruin a cat by dumping too much raw fuel into the exhaust. Some common items that can cause this issue would be faulty fuel injectors, a bad ignition coil, fouled spark plugs and a leaking fuel pressure regulator.

A flashing check engine light indicates a catalyst damaging misfire and this can damage a cat very quickly. In this situation, the engine should be shut off immediately, and the car should be towed to a repair shop for diagnosis.

It is very important that the cause of the cat failure is determined before a new cat is installed. Failure to do so may ruin a new catalytic converter, and the warranty may not cover the damaged part.

» MORE: Get a catalytic converter replacement quote

Catalytic converter location

Beware of catalytic converter theft

Catalytic converters are often stolen because they contain expensive materials. A thief can remove a cat in a matter of minutes, so the police should be called immediately as the thieves may still be in the area stealing more of them. A sign of a missing catalytic converter is a very loud exhaust note, and some insurance policies will cover the replacement.

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