How to really tell if catalytic converter is bad

I measured if with my laser thermometer. One converter was about 250 inlet and 300 outlet. Other is 250 inlet and about 550 outlet. I’m being told it could simply be spark plugs. Is this true?

Take it to a good mechanic. I don’t know what test you were doing or what it proved.

Sparkplugs?? Really?? Where are you getting your information from.

Most of the time diagnosed bad catalytic converter is just a bad O2 sensor.There are tests that you can run to determine if the cat is bad.

If you had the equipment and expertise you could bore holes in the pipes and test the gasses going in and the gasses going out with a combustion analyzer… but that would be expensive overkill and I’ve never actually heard of anybody going to that length. Might be interesting, however. Especially if you could get the actual operational specs for the converter. By volumetrically controlling/measuring the exhaust gas flow and temperatures, and reading the difference in the oxygen levels, carbon monoxide levels, carbon dioxide levels, and hydrocarbon levels, you could get a pretty good profile and conceivably measure the condition of the converter beyond just pass/fail.

However, a knowledgeable tech can monitor the signals out from the upstream and downstream oxygen sensors and get a pretty good idea what’s going on. If one of the sensors has a signal not normal (such as a flatline) he’d know the sensor is bad. If the signals are normal but not showing the difference in oxygen level that the converter should be causing, he’ll suspect that the converter catalyst is contaminated (converter is shot).

When my cat went out, I got a code. The way the dealer checked it was to clear the code and when the cat code came back again in a few minutes, they replaced it. The only other cat I’ve had that went bad fell off due to rust. Normally if there is no misfiring or other abuse, they last a long time.

To quickly/easily determine if a catalytic converter is bad, you take either a scanner that has real-time monitoring capability or a scope meter and measure the voltage from the post cat O2 sensor.

The cat is designed to absorb and store any oxygen in the exhaust gas. So the post cat O2 sensor voltage should read steady between 500-700 Mv’s. If the voltage of the post cat O2 sensor begins to fluctuate, it means the cat efficiency has fallen off where it can no longer absorb and store all the oxygen in the exhaust gas.


The platinum-palladium coating in the catalytic converter core only separates the oxygen atoms from the nitrous-oxide molecules created when the nitrogen in the atmosphere bonds with the oxygen in the atmosphere under heat and pressure. The oxygen atoms are then available for the passing carbon monoxide molecules to catch and become carbon dioxide, and (with the heat of the cat converter) unburned hydrocarbons can separate and bond to (known as the “second burn”), forming H2O and CO2. The converter absorbs and stores nothing. By definition, a catalyst causes change without itself changing.

Changes to the performance of the catalyst are caused by contamination of the catalyst that interferes with the ability of the NOx, CO, and HC molecules to directly come in contact with the platinum-palladium. The chemical conversion only takes place with molecular contact.


Excellent link, but I cannot agree with the description of the catalyst “storing” oxygen. To do so would require that the platinum-palladium oxidize. “oxygen Saturation” is that point where the catalyst has reached its maximum capacity to strip the oxygen from the NOx. That isn’t based on retention of the oxygen atoms, it’s a simple flow capacity issue. If the catalyst reaches “saturation”, the excess oxygen passes through not because the catalyst has absorbed oxygen, but rather because the converter cannot process the excess volume effectively. Eliminate the hydrocarbon-rich source and the catalyst will return to normal, never having absorbed anything.

However, we’re admittedly debating a technical/chemistry issue beyond the needs of the OP. I do support your posted protocol and am in full agreement with the path the OP should take.

Then one would have to ask themselves, “Why do they use an oxygen sensor to monitor catalyst efficiency?”


Because you’re monitoring the free(d) oxygen output of the converter.
Ask yourself this, “if the catalyst stores oxygen, would not a good catalyst capture and store the free oxygen in the exhaust gasses, putting a lot less out the other end than goes in?”.

You might also ask yourself “if the catalyst captures and stores the free oxygen atoms, where does the oxygen come from that converts carbon monoxide to carbon dioxide and unburned hydrocarbons into carbon dioxide and H2O?”.

Excellent link, but I cannot agree with the description of the catalyst “storing” oxygen.

Then what is the purpose of the cerium in catalysts?

Cerium encourages the platinum-palladium to strip the oxygen atoms from the NOx, but I don’t believe it stores the oxygen for future release, although I do acknowledge that cerium readily oxidizes. Oxidization, however, is a chemical change that can only be reversed through resmelting. Once cerium has oxidized, the oxygen bonded within it is not available for use by passing molecules that need another oxygen atom.

And now I’m going to readily admit that I would welcome the input of a knowledgeable graduate chemist.

I also admonish the OP not to consider this debate to be germane to his/her problem. We’re drifting off on a tangent here. An in depth understanding of the chemistry of a catalytic converter isn’t necessary to diagnose and fix your vehicle. Tester has given you excellent advice to accomplish that. I love and learn from these debates, but feel free to ignore them.

"When my cat went out I got a code"
What is the code for for that? I have never been able to find it.

TSM has a pretty good handle on the operation of the catalytic converter. The converter does not store oxygen, it combines it with the unburned hydrocarbons and CO to make CO2 and H2O. The upstream O2 sensor should see higher concentrations of O2, once the gases hit the catalyst the carbon (CO, unburned fuel) combines with the oxygen and the downstream sensor should see lower amounts of O2. This is not they way you would measure the efficiency of the converter in a lab, but laboratory devices would never last more than a few minutes under a car. Using O2 sensors is an elegant way to make a practical and reliable system.

The oxygen storage test is a layman’s way of checking the efficiency of the converter, the average mechanic can follow the steps and the description makes sense. It is not the correct chemistry, but it makes testing easy and it works.

Yes, oxygen is stored in the body of the catalytic converter.
It’s stored in the gases during their residence time, streaming past the catalyst coated surfaces.
Under light load cruising conditions (like when the catalyst monitor is run) that time might be a few 10ths of a second.

I like your temperature difference method…If one CAT is hotter than the other, it’s probably working harder than the cooler one. A slight misfire in one cylinder could cause this, along with other things…year, make, model and mileage of your vehicle would be helpful…

.If one CAT is hotter than the other, it's probably working harder than the cooler one.
Has it been conclusively determined that the cats are in parallel? If they are in series, it would make perfect sense for the downstream cat not to get as hot as the upstream one. (AFAIK, year/make/model are all a mystery.)

Circuitsmith, I can accept that definition. It’s a stretch of the word “stored”, but I can live with it.

Having read constant debates over many years on this board on this topic, my advice is rather simple. DO NOT REPLACE A CATALYTIC CONVERTER UNTIL YOU HAVE REPLACED THE BEFORE AND AFTER SENSORS AND DRIVEN IT A WHILE. PERIOD.

I don’t care what the scanner says, and I don’t care what the service manager says.

Over many years, we have had professional mechanics say they never have to replace the cat, the sensors always fix it, unless there is physical damage or car is massive oil burner And, others say they always have to replace the cat. It doesn’t take much thought to know who is right.

Of course, my opinion is partly based on the prices they charge for replacing Toyota cats. Sensors are a drop in the bucket.

One some vehicles these O2/air-fuel ratio sensors can cost up to $150.00 per sensor.

So what you’re saying is, even though there is no O2 sensor codes indicating there’s a problem with the O2 sensors, replace them anyway. Because you want to shoot the messenger?

That’s not diagnostics. That’s throwing expensive parts at the vehicle when it’s evident what the problem is.