Bad or Clogged Catalytic Converter Symptoms: How to Tell

Bad catalytic converter symptoms point at one of two faults. A converter that stopped cleaning the exhaust only turns on the check engine light, while a blocked one costs you power on the first on-ramp.

Many of the same signs also come from cheaper faults, like a sensor or a misfire. So sort what you notice before you buy what EPA calls a very expensive part to replace.

What do bad catalytic converter symptoms mean?

They point at one of two separate failures of the converter, a ceramic honeycomb coated in precious metals that clean the exhaust. The two aren't degrees of the same thing:

• It stops converting. The honeycomb is intact and gas flows normally, but the coating has stopped cleaning the exhaust. Only the oxygen sensor behind it, which reads what comes out, notices, so all you see is the check engine light.
• It stops flowing. The honeycomb has broken up, melted or filled in, so the engine exhales through a restriction, and every symptom that follows comes from that.

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Each common symptom points at one of the two, a converter that stopped converting or one that's blocked. Sorting it is the step that decides what you do next.

Can you drive with a bad catalytic converter?

If the light is the only sign, the car drives normally and the converter itself does no harm, though it may fail an emissions test, unless your car is on a converter recall such as the Soul's, where the heat can damage the engine. But what killed it, a misfire, burning oil or a coolant leak, may still be running, so find it before you plan the repair.

If it's a blockage, MagnaFlow, a converter maker, notes the restriction makes the engine strain and, in severe cases, interferes with how it runs. We'd stop driving and get it checked if:

• The car won't pull above town speed. The sign of a restriction, or of a misfire.
• The check engine light is flashing. It points to an active misfire, which can overheat the converter.
• You see a glow or smell burning under the car. Heat is building where it shouldn't, and a burning smell has its own causes.

What does a clogged catalytic converter feel like?

A clogged catalytic converter feels like an engine that runs out of breath: fine around town, then weak and slow to respond when you merge onto a highway or climb a grade. It also strains and burns more fuel, pushing its exhaust through the blockage.

Heat is the other sign, because trapped exhaust can make the converter glow red under the car. A bad blockage can also make the engine hard to start or run poorly, MagnaFlow says.

What if the light is on and nothing else?

If the car drives as it did and the only new thing is the light, the converter has most likely stopped converting, with nothing blocked. The computer compares the oxygen sensors ahead of and behind it, and when they read too alike it stores P0420, or P0430 for the second side of a V6 or V8.

That code is a measurement, not a diagnosis, because a tired converter, a failing rear sensor or an exhaust leak can each set it. Our P0420 code page explains what the test can prove, and some carmakers fix it with software before any new converter.

What do the rattle and the smell mean?

The bad catalytic converter sound is a rattle, like gravel in a can, from the ceramic honeycomb broken into pieces inside the steel shell. A loose heat shield can rattle too, and our page on a rattling noise when accelerating covers the other sources.

The rotten egg smell is sulfur the converter isn't processing, MagnaFlow says, though a sulfur smell alone doesn't prove the converter failed, as our rotten egg smell guide explains. MagnaFlow also lists a too-rich fuel mixture among what gets into the exhaust and harms a converter, so check the fuel mixture before buying one.

A broken converter honeycomb

What gets mistaken for a bad catalytic converter?

A failing oxygen sensor behind the converter, an exhaust leak ahead of the rear sensor, a misfire, and a recall or engine fault all get mistaken for a bad catalytic converter. Some of them also kill converters, so a new one fitted behind the same fault fails the same way.

Here are the four impostors and causes to rule out first, from the cheapest check up:

1. The oxygen sensor behind the converter

A failing sensor behind the converter can set the same light-only code over a perfectly good catalyst. A shop can watch both sensors live on a scan tool, and Walker lists sluggish sensors among the causes of converter failure, so a lazy one can be cause and impostor at once.

2. An exhaust leak ahead of the sensor

A leak before the rear sensor lets outside air in, so the sensor reads extra oxygen, called a lean exhaust, and the code can follow with the converter doing its job. Ask the shop to check the joints and gaskets ahead of the rear sensor before anyone quotes a converter.

3. A misfire

A misfire can kill the converter: EPA's warranty fact sheet traces skipped spark plug changes to misfiring and eventual converter damage, and Walker names misfires among the causes. It also causes rough idle and lost power, which looks like a blocked converter from the driver's seat.

That's why a flashing check engine light is more urgent than a steady one. The flashing check engine light page explains why to ease off, and fixing the misfire comes before any converter.

4. A recall or an engine fault

On some cars the converter trouble is part of a known engine problem that the carmaker already covers. The Kia Soul's recall, in the cards below, pays for the converter and, if needed, the engine, because high exhaust heat can damage both.

That recall explains why the same Kia turns up in so many converter complaints. Check the VIN, the 17-character vehicle identification number, before you buy a converter, because a recall or warranty fix may already cover it.

How do you test a catalytic converter?

For the light-only failure, the test is a scan tool, with a shop watching the oxygen sensors ahead of and behind the converter live. The same reading shows a lazy rear sensor, and the joints ahead of it get checked for leaks.

For a blockage, MagnaFlow has the exhaust measured just before and just after the converter with an infrared thermometer, engine warm. On a working converter the outlet usually reads hotter, so ask the shop for both readings.

MagnaFlow adds an engine vacuum test for a car built before 1996 or with no check engine light. On any car, a shop can also measure exhaust back pressure with a gauge at the front oxygen sensor port, so ask which test it ran before you pay.

Can a clogged catalytic converter be unclogged?

Sometimes, if the blockage is soot rather than melted or broken ceramic, and only after whatever caused the soot is fixed. Our page on whether catalytic converter cleaners work finds a working mechanism for only two of the six ways a converter fails.

Once the honeycomb has physically broken up or fused, nothing poured into the tank reaches it or repairs it. A rattle or a glowing converter means it's past cleaning.

Who might owe you a converter already?

Under federal law, a converter on a 1995 or newer car or light truck is covered for 8 years or 80,000 miles from delivery, whichever comes first. The cover is for a defect, and EPA says a carmaker can refuse a claim if poor maintenance, like skipped spark plug changes, caused it.

If a claim is refused, EPA tells you to ask for the reason in writing, with the names of the people who decided. Then appeal to the carmaker until you've used every step.

Past that, a shop may fit an aftermarket converter only when the original is missing, damaged or truly worn out, and never while a recall or warranty still covers it. A new aftermarket converter must carry its own warranty, and a used or reconditioned one needs none.

In California, an aftermarket converter needs approval from CARB, the state's air board, for your specific car. A used converter can't be legally sold or installed there at all.

A recall can owe you one as well, and some carmakers replace the converter free on the cars on their VIN list. Run the VIN before you pay: our free VIN checker lists the recall campaigns for your car's model year.

What should you do about a bad catalytic converter?

Work from the symptom, not the part, and leave the converter until last. To sort out bad catalytic converter symptoms, start by deciding which failure you have, and the first three steps cost little or nothing:

1. Decide which failure you have. The symptom list in the first section sorts each sign into one of the two.
2. Read the code. Any scan tool reads it, and it tells you whether the computer is reporting the converter, a sensor, a misfire or the fuel mixture.
3. Run the VIN and check the warranty. A dealer checks recalls and campaigns by VIN, as the warranty section above shows, and the federal warranty may pay.
4. Rule out the impostors. Ask the shop to check the rear oxygen sensor, exhaust leaks ahead of it, misfires and the fuel mixture, then to test the converter itself.
5. Fix what killed it, then replace it, or have the carmaker's procedure done. A new converter fitted behind a live misfire is a repair you'll buy twice. Some carmakers first have a set drive cycle run, a fixed pattern of driving that retests the converter.

If the code comes back after a new converter, ask what was fixed ahead of it in the engine. The cost of a replacement, part by part, is on the catalytic converter replacement cost page.

Common questions

Asked most often

What happens when a catalytic converter goes bad?

One of two things. Either it stops converting, and the check engine light comes on while the car drives normally, or it stops flowing, and the engine loses power when you ask for it.

How long will a car run with a clogged catalytic converter?

There's no set time or mileage, because it depends on how bad the blockage is. MagnaFlow says that in severe cases the restriction can interfere with how the engine runs, so a car that won't pull should be checked rather than driven.

Is a catalytic converter covered by warranty?

If the car is within 8 years or 80,000 miles of delivery, whichever comes first, federal law may cover it as a major emission control component, though EPA says a carmaker can deny a claim if skipped maintenance caused the failure. A recall can also cover it, so check the VIN.

This article is general information, not advice about your specific vehicle. See the disclaimer.