A catalytic converter can potentially become internally restricted.

If exhaust gases can't flow through the exhaust system properly, engine performance may suffer.

You might notice:

• Sluggish acceleration

• Difficulty reaching higher speeds

• Reduced power

• Poor performance under load

• Engine feeling like it can't breathe

The problem may become more noticeable during:

• Highway acceleration

• Climbing hills

• Heavy throttle

• Higher engine speeds

A severely restricted converter can make a vehicle feel dramatically underpowered.

But reduced power has many possible causes.

Fuel delivery, ignition, turbocharger systems, transmission problems, airflow sensors, restricted intake, and engine mechanical problems can create similar symptoms.

Test before replacing.

Catalytic converters contain an internal catalyst structure.

If that internal material becomes damaged or breaks apart, you may hear a metallic rattle.

It may be most noticeable:

• During startup

• At idle

• When revving the engine

• While accelerating

• When driving over bumps

But not every exhaust rattle comes from the converter.

Possible alternatives include:

• Loose heat shield

• Broken exhaust hanger

• Muffler damage

• Exhaust pipe contact

• Other loose components

Have the source of the noise identified.

Changes in exhaust odor can indicate combustion or emissions-system problems.

Some drivers describe certain exhaust problems as producing a sulfur-like or rotten-egg smell.

However, smell alone isn't a reliable catalytic-converter diagnosis.

Unusual exhaust odor can also be influenced by:

• Fuel mixture

• Engine operating condition

• Fuel quality

• Misfires

• Emissions-system problems

And remember from our exhaust-leak discussion:

Never use smell as a way to determine whether exhaust gases are safe.

Carbon monoxide itself is odorless.

If exhaust gases appear to be entering the passenger compartment, treat that as a safety concern.

If the engine or emissions system isn't operating correctly, fuel economy may decline.

You may notice you're visiting the gas station more frequently even though your driving habits haven't changed significantly.

But fuel economy is affected by many factors:

• Driving habits

• Tire pressure

• Weather

• Traffic

• Idling

• Engine problems

• Fuel-control issues

• Maintenance condition

Declining fuel economy by itself doesn't prove the catalytic converter is bad.

Combined with other symptoms, however, it can provide another clue.

Catalytic converters naturally operate at high temperatures.

But certain engine problems can cause converter temperatures to become abnormally high.

One important example is a severe misfire.

If unburned fuel enters the exhaust system, it can burn inside the catalytic converter and generate extreme heat.

This can damage the catalyst.

Potential warning signs may include:

• Burning smell

• Excessive heat underneath the vehicle

• Discoloration

• Severe performance problems

• Flashing Check Engine light

A flashing Check Engine light deserves prompt attention because it can indicate a serious misfire that may damage the catalytic converter.

Don't continue driving a severely misfiring vehicle just because it still moves.

A failing catalytic converter may contribute to excessive emissions.

Depending on where you live and the testing requirements, the vehicle may fail an emissions inspection because of:

• Stored diagnostic trouble codes

• Catalyst-monitor status

• Excessive emissions

• Check Engine light

• Other emissions-system problems

But an emissions failure still requires diagnosis.

The goal isn't simply to make the warning light disappear.

The goal is to determine why the emissions system isn't operating correctly.

What Does a Catalytic Converter Actually Do?

Your engine produces exhaust gases during combustion.

Those gases can contain pollutants such as:

• Carbon monoxide

• Hydrocarbons

• Oxides of nitrogen

The catalytic converter uses specialized catalyst materials and high operating temperatures to promote chemical reactions that convert much of these pollutants into less harmful compounds.

It does this continuously while the engine is operating under appropriate conditions.

That's why the converter is such an important part of modern emissions systems.

Catalytic converters are installed in the exhaust system.

Depending on the vehicle, there may be:

• One converter

• Multiple converters

• Converters integrated near the exhaust manifold

• Additional converters farther downstream

Some modern vehicles position converters close to the engine so they reach operating temperature quickly.

This can make replacement labor and parts costs vary considerably.

Converters use specialized catalyst materials.

Depending on design, these can include precious metals such as:

• Platinum

• Palladium

• Rhodium

Vehicle-specific emissions requirements, converter design, manufacturing, and certification can also affect replacement cost.

That's why a catalytic converter shouldn't be replaced casually based on a trouble code alone.

P0420 generally indicates that the engine computer has determined catalyst-system efficiency on Bank 1 is below the expected threshold.

P0430 generally refers to the corresponding condition on Bank 2 on applicable engines.

The computer may use information from oxygen sensors located before and after the converter to evaluate catalyst performance.

If downstream sensor behavior suggests the converter isn't storing and processing oxygen as expected, the computer may set a catalyst-efficiency code.

But several factors can influence those readings.

The code identifies a condition—not necessarily the failed component.

This is where diagnosis becomes important.

Oxygen sensors provide information the engine computer uses for fuel control and catalyst monitoring.

A sensor problem can potentially create misleading data.

But replacing oxygen sensors automatically because a catalyst code appears isn't good diagnosis either.

A technician may evaluate:

• Upstream oxygen-sensor behavior

• Downstream oxygen-sensor behavior

• Fuel trims

• Exhaust leaks

• Misfire history

• Related trouble codes

• Converter temperature

• Engine operating condition

The goal is to use the data together.

Suppose a cylinder misfires.

The air-fuel mixture may not burn properly inside that cylinder.

Some unburned fuel can enter the exhaust.

When it reaches the extremely hot catalytic converter, additional heat can be generated.

Persistent severe misfires can overheat and damage the catalyst.

That's why replacing the converter without repairing the misfire can be an expensive mistake.

You install a new converter.

The original engine problem remains.

Then the replacement converter becomes damaged too.

Fix the cause before replacing the casualty.

Potentially.

An engine that burns significant amounts of oil can send contaminants through the exhaust system.

Over time, contamination may reduce catalyst effectiveness.

If a failed converter is found on an engine that consumes excessive oil, the oil-consumption problem should be considered during diagnosis.

Otherwise, the replacement converter may eventually face the same conditions.

Internal engine problems that allow coolant into the combustion chamber can also contaminate the exhaust stream.

For example, certain head-gasket or cylinder-head problems may cause coolant to enter combustion.

That can potentially damage emissions components.

Again:

Don't only ask why the converter failed. Ask what caused it to fail.

Potentially, depending on leak location and severity.

An exhaust leak can influence oxygen content in the exhaust stream and affect sensor readings.

That's why the exhaust system should be inspected when diagnosing certain emissions codes.

Replacing a converter while ignoring an upstream exhaust leak can lead to unnecessary expense.

A significantly restricted converter can make the engine feel like it can't exhale.

You may notice:

• Poor acceleration

• Major power loss

• Difficulty reaching higher RPM

• Engine struggling under load

• Excessive heat

• Possible stalling in severe cases

Technicians have several ways to investigate an exhaust restriction.

Don't diagnose restriction based solely on how the vehicle feels.

Depending on the symptoms and vehicle, diagnosis may include:

• Scanning diagnostic trouble codes

• Reviewing oxygen-sensor data

• Checking fuel trims

• Looking at misfire counters

• Inspecting for exhaust leaks

• Checking converter temperature

• Testing for exhaust restriction

• Inspecting the converter physically

• Checking engine performance

• Evaluating emissions-monitor data

No single test is ideal for every vehicle.

Good diagnosis considers the entire engine and emissions system.

A catalytic converter is downstream from the engine.

That means the exhaust entering the converter reflects what's happening during combustion.

If the engine has:

• Persistent misfires

• Excessive oil consumption

• Improper fuel mixture

• Internal coolant leakage

the converter may suffer the consequences.

Replacing it without fixing the engine problem can mean replacing it again later.

You'll find products claiming to clean catalytic converters.

Be careful about treating these as a substitute for diagnosis.

A chemical product cannot repair:

• Melted catalyst material

• Broken internal substrate

• Severe physical damage

• Cracked converter housing

• Major exhaust restriction

If a Check Engine light indicates catalyst efficiency problems, determine the actual cause before relying on an additive as a repair.

That depends on the type and severity of the failure.

A vehicle with a catalyst-efficiency code but otherwise normal operation presents a different situation from one with:

• Severe power loss

• Major exhaust restriction

• Overheating converter

• Flashing Check Engine light

• Engine misfire

• Stalling

If the vehicle is severely losing power or the converter appears to be overheating, don't continue normal driving.

Have it inspected.

Stop driving or seek prompt professional assistance if:

• The Check Engine light is flashing

• The engine is severely misfiring

• The vehicle suddenly loses substantial power

• The exhaust system becomes excessively hot

• You smell something burning

• The engine stalls repeatedly

• Exhaust fumes appear to be entering the cabin

These symptoms may indicate problems beyond simple catalyst efficiency.

There's another reason your vehicle may suddenly become extremely loud:

The catalytic converter may have been stolen.

Converter theft typically creates a dramatic exhaust noise immediately after startup.

You may also see:

• Cut exhaust pipes

• Missing exhaust section

• Hanging components

• Check Engine light

That's very different from gradual internal converter deterioration.

If the vehicle suddenly sounds like it has an open exhaust overnight, inspect for theft or physical exhaust damage.

Because catalytic converters can be expensive, ask:

What trouble codes were found?

What testing confirmed the converter has failed?

Were the oxygen sensors evaluated?

Was the exhaust checked for leaks?

Does the engine have any misfires?

Were fuel trims checked?

Is the engine burning oil or coolant?

Is the converter restricted or simply below efficiency threshold?

What caused the converter to fail?

Does the replacement meet the emissions requirements for my vehicle and location?

What warranty comes with it?

These questions can potentially save you from replacing an expensive component without correcting the actual cause.

A failing catalytic converter may produce several warning signs:

Check Engine light.

Reduced engine power.

Rattling underneath the vehicle.

Unusual exhaust smells.

Reduced fuel economy.

Excessive exhaust-system heat.

Emissions-test failure.

But the most important lesson is:

A catalyst code isn't automatically a catalytic-converter diagnosis.

The converter may have failed.

Or another engine or exhaust problem may be influencing the system.

And even if the converter has failed, you still need to know why.

Read the codes. Analyze the data. Check the engine. Inspect the exhaust. Confirm the failure. Repair the root cause.

Check Engine Light or Losing Power?

FixFlowIQ makes it easier to compare your diagnostic and repair options.

Describe exactly what your vehicle is doing — Check Engine light, reduced power, unusual exhaust smells, rattling, poor acceleration, or emissions problems — and submit your repair request.

If you already have diagnostic trouble codes such as P0420 or P0430, include them with your request.

Vetted automotive professionals can review your symptoms and provide repair options for you to compare.

Understand the diagnosis. Compare your options. Choose the provider that's right for you.

Because when an expensive catalytic converter is involved, you don't want someone simply clearing a code and replacing a part.

You want to know what failed—and why.