Catalytic Converter – Knowing The Basics – Failure Signs And Testing

Catalytic Converter - Knowing The Basics - Failure Signs And Testing
Catalytic Converter - Knowing The Basics - Failure Signs And Testing

So, a catalytic converter, is a device used to reduce the emissions; from an internal combustion engine.

Consequently, the catalytic converter works, by breaking down unburned gases; left over by the combustion process. But, requires you to use, unleaded fuel.

This is because, the lead in conventional fuels, “poisons” the catalyst; preventing the catalytic converter from working on the pollutants. And, they only work at high temperatures; when the engine has had chance to warm things up.

Above all, without a catalytic converter, your vehicle will no longer be filtering and reducing harmful emissions; including hydrocarbons, nitrogen oxides, and carbon monoxide.

Knowing The Basics

The exhaust from the engine, gets filtered through this device; so harmful chemicals can be removed, like:

  • Carbon monoxide (a poisonous gas)
  • Nitrogen oxides (a cause of smog and acid rain)
  • Hydrocarbons (a cause of smog)

But, a bad oxygen sensor (O2) sensor, can feed wrong information to the computer; which can affect the performance of catalytic converter.

Consequently, too rich a fuel mixture, causes unburnt fuel to enter the catalytic converter. So, at some point it will be damaged, due to thermal shock.

Catalytic Converter Break Down
Catalytic Converter Break Down

So, inside the converter, the gases flow through a dense honeycomb structure. Consequently, made from a ceramic and coated with the catalysts. In addition, by using a honeycomb structure, the area is greatly increased.

Check Engine Light And Codes

The check engine light (CEL) or malfunction indicator light (MIL), is your best friend; when dealing with emission problems.

Check Engine Light From Bad Catalytic Converter
Check Engine Light

So, whenever this light comes on, check for trouble codes. The common catalyst failure codes are, P0420 or P0430. But, check your code reader’s manual to be sure.

Signs Of Your Catalytic Converter Failing

There are a handful of telltale signs, your catalytic converter is failing:

  • Poor acceleration, when stepping on the gas pedal.
  • Misfiring.
  • Noticeably, less fuel efficiency.
  • Dark, sooty smoke exiting your muffler.
  • Engine starting trouble.
  • Failing an emissions test.
  • A pungent sulfuric or rotten egg smell.
  • A Check Engine light, in conjunction with any of these other symptoms.

Catalytic Converter Testing

An effective way to test, for a blocked or clogged catalytic converter, is with a vacuum gauge.

Doing The The Vacuum Test

Above all, an exhaust system restriction, will cause a loss of engine power. But, so can many other problems. So, if you suspect a plugged-up converter, you need to test the system. This test requires a vacuum gauge. So, if you don’t have one, consider buying one.

Doing The The Vacuum Test
Doing The The Vacuum Test

A vacuum gauge is a useful diagnostic tool, that will come in handy in other repair projects:

  • Set the transmission to park (automatic) or neutral (manual), and apply the parking brake.
  • Disconnect the vacuum hose at the brake power booster; connect the vacuum gauge to the hose or to another direct intake manifold port.
  • Start the engine and let it idle for about 15 to 20 minutes; so that it reaches operating temperature.
  • At idle, vacuum reading should be, between 18 and 22 in-Hg (inches of Mercury).
  • Increase and hold ending speed to about 3000 rpm. Vacuum reading should drop. But, should go back to the previous level in a few seconds. If not, it’s likely the exhaust system is blocked. Continue to the next step.
  • Repeat the test, but this time, snap the throttle valve open about four times; rising engine speed to about 2500 rpm. Take note of the vacuum reading, with each snap of the throttle.
  • So, if you see that gauge needle, dropping steadily to near zero; most likely the catalytic converter or muffler is restricted.

  • To confirm a possible restriction; loosen the back pipe from the catalytic converter; just enough to allow gas to flow through.
  • Repeat step 6. If vacuum doesn’t drop considerably this time; you’ve found the blockage (back pipe or muffler). Otherwise, go to the next step.
  • Loosen the header exhaust pipe from the converter; just enough to allow exhaust gas to flow through.
  • Repeat step 6 and take note of your vacuum readings. If your vacuum readings show normal vacuum; the converter is restricted or clogged up.

Other Testing Options:

  • Loosen the catalytic converter, from the header pipe or exhaust manifold.
  • This allows gases to flow through. If you notice an improvement in engine performance; you are on the right track.
  • Just like the vacuum test above; a back pressure test can help you diagnose a clogged exhaust system.
  • This test is done directly at the exhaust system; to detect a plugged catalytic converter or muffler or pipe.
  • This simple test, requires a back pressure gauge.
  • The temperature test, using an infrared thermometer.
  • In a good catalytic converter; the rear of the converter will be, 20 degrees hotter than the front.
Temperature Test Using An Infrared Thermometer
Temperature Test Using An Infrared Thermometer

Knowing The Basics

In most cases, the catalytic converter will last; for the entire life cycle of a vehicle. While sturdy, the catalytic converter can slowly fail over time; as the catalyst elements wear out. But, high mileage is not the only culprit; as failure could be due to, an improperly operating engine.
Burnt Exhaust Valve From Bad Catalytic Converter
Burnt Exhaust Valve

Worn spark plugs, or burned and leaking exhaust valves; allow unburned fuel into the exhaust system. Consequently, it ignites in the catalytic converter and melts the internals. A weak ignition system, can also cause the same problem; by not getting enough spark to the plugs.


So, catalytic converter problems, could also be something more serious. For instance, like heavily worn piston rings or a jumped timing chain. Consequently, these problems allow fuel or air; into the wrong place at the wrong time. Furthermore, a head gasket or intake manifold leak, can cause problems as well. Finally, oil or coolant can coat the catalyst; setting off a check engine light.

Thank You !