Catalytic converter

My wife’s car is a 2002 Prius, 149 k miles. Lately the check engine light has been coming on. I took it to the dealership, and their assessment is that the catalytic converter needs to be replaced. They are recommending about $2,000 + of repairs.
While I am contemplating the practicality of investing that much in keeping this car going I’m wondering if it harmful or dangerous to continue operating this vehicle.

It is harmful to the environment in that you will be emitting more pollutants in the exhaust, but it is probably not harmful to the car if it is running well. Ultimately the cat may begin to degrade, and if it gets plugged up, you run the risk of overheating with the associated problems that can cause. You might want to see a reputable independent mechanic who can use a reputable aftermarket product - less expensive, but still not exactly cheap. If you live where emissions checks are required, your car will not pass until the cat is replaced.

Look for a good indy shop here:

'Round here catalytic is pronounced "Cadillac"
A takeoff from the post ahead of this, about the pronounciation of “kilometer”

I’d Definitely Want To Know Why The Catalytic Converter Went Bad, If It Actually Did. Many Things Relating To The Converter Can Set A “Check Engine” Light And Codes That Relate.

A light and code(s) for a deficient converter are some of the most frequent car problems people have. Converters usually are the suspect culprits and many converters are replaced only to have that pesky light come right back because the real cause wasn’t corrected. Wouldn’t that tick you off ?

I’d have them assure me that their assessment is correct or you won’t be paying for it. See if they are that sure of their diagnosis.


“I’m wondering if it harmful or dangerous to continue operating this vehicle.”

I suppose if the converter is really bad / going bad then it could do some engine damage and / or overheat and start a fire.

The best thing you could do is to get the actual DTCs (diagnostic trouble codes) and post them here. Perhaps they’re on your repair order. They will be like PO402 or PO420 or PO123, etcetera.


CSA makes a good point, but at 149K it isn’t out of the question for the catalytic converter to have gone bad. Still, getting the code(s) would be a good idea - we can tell you what else to ask them to check.

We Run Our Cars For 200,000 To Over 300,000 Miles (Not Subarus). I’ve Never Replaced A Catalytic Converter. I Don’t Consider Them To Be Maintenance Items.


I wonder how driving style factors into converter life.
Often I see drivers who operate their vehicle in an abrupt manner, using the throttle like a toggle switch.
I imagine when the throttle is stomped on the engine is more likely to put out a puff of hydrocarbons, making the converter work harder.
Higher but ‘acceptable’ oil consumption, 1 qt. per 1-2000 miles, probably doesn’t help either.

My 1988 Accord passed the treadmill emissions test by a wide margin at 200k+ with the original converter and O2 sensors.

My colleagues who work on the Catalytic converters tell me that they do not fail on their own. They need the following in order to fail.

  • regular short thermal cycles (very short stop and go trips)
  • Higher than normal oil consumption
  • Contamination (bad fuel or coolant in exhaust)
  • Fuel pooling in the CAT and then getting lit off.

Google has a lot on this failure. My cat showed a OBDII failure some months ago, after engine braking in second nearly 17 miles on the mile drop between Puebla and Orizaba here in Mexico. I did a lot of reading. Some weeks later, my light went off, and it is okay now. Someone, I forget, suggested it may have picked up just enough oil in that 17 miles (the computer shuts off the fuel to the injectors when it is coasting down hill) to clog up the cat screen, and it eventually burned itself clear.

The computer actually cannot measure directly cat efficiency. It checks composition of the exhaust before the cat, and after. When the car is cold, the difference will be small, because unburned hydrocarbons will pass through the cat.

Once the cat is hot, the expensive metal screen inside it burns up any hydrocarbons that enter it, not yet burned.

If there is not enough difference before and after the cat when the cat is hot, the computer calls it low efficiency. Bad sensors can confuse the computer, of course.

There is a car talk show host in Houston, named Scott Kilmer, who has a video on Youtube on this very issue, in which he says he has for years put a gallon of lacquer thinner (note I said lacquer thinner, not lacquer, heh, heh) in his half-full gas tank, then drives it out, and swears it cleans the cat, though he says it is better to take the cat off if you can, and toss it in a bucket of cold water with laundry soap in it overnight. Problem is the cats get rusted on.

The folks here say the lacquer thinner can’t work, and will wreck your fuel system. I Googled a lot, and learned those who have never tried it all 100% say it can’t work and will destroy your car. The few who have tried it all say 100% it worked. One person said very old Hondas, pre 2000, can have fuel system damage.

Anyway, I may well try it if mine fails again, but whoever tries it pays for damage if any, without whining about it. That is what I will do.

I encountered several mechanics on other boards who said ALWAYS when they get that code, the sensors are the problem. I have no opinion.

A bad post converter 02 sensor could be sending a bad reading to the ECU making it throw a code for a bad converter.

“Someone, I forget, suggested it may have picked up just enough oil in that 17 miles”

And I hypothesized that the converter simply cooled off due to the fuel cutoff.
The computer doesn’t test for converter efficiency while the engine warms up, but in this case maybe the engine block was still warm but the converter ice cold.