Is it really the catalytic converter?



I can make the Check Engine Light come on if I drive at 48 mph. This clears after several re-starts.

Several garages say that the catalytic converter is bad, and that’s based on the code that’s set, as well as tests of the O2 sensors.

This diagnosis does not explain why the Check Engine light resets itself. The shop information shown in “All Data” states that the Check Engine light will reset after 10 restarts. So, that’s what I have: a mostly happy computer.

In my part of New York, they just read the computer for codes, and that’s the emissions test. If it says “it’s OK,” then we pass emissions. I’m looking for a shop that can actually measure tail pipe emissions. If I have an emissions issue, I’ll get a new converter. But, $1200 for a converter is a lot of money. As noted, I’m skeptical that this is the issue. The last time they claimed the catalytic converter was bad, a new thermostat fixed the dual issue of “warms up too slowly” and “catalytic converter bad.”


You are on the right track by wanting to actually analyze the gas out of the tailpipe.

I am speaking of having a shop sniff your tailpipe not the emissions test station. I conclude you want to know how efficient your cat is. There are 3 tests for a cat. One is to check for at least 100F difference in temp just ahead and just behind the cat. The other two involve using a gas analyzer and are a little more complicated.


You didn’t say what year your car is. If it is a 1996 or newer, I don’t believe their emission machines will let the mechanic (legally) probe the tailpipe for an emissions test. That is the year OBDII became universal. There are a few 1994, and some more 1995 cars out there with OBDII, but the computer in the machines will detect the model year car the mechanic inputs (or scans) and use that for setting itself up to perform the test. Now the shop should be able to probe your tailpipe in a “troubleshooting” or “diagnosis” mode, I’m not even sure about that, I’m not sure what type of machines other states use besides my own. But if they can do that, it will give you an indication of how clean your car is burning. If your car has very many miles on it, you might be able to change plugs, filters, maybe wire, oil, and get it to pass simply by getting it to run more effecient.


How about make model and year as well as the number of miles on that Model A?

It is normal for most systems to rest the code when it does not re-occur for X number of driving cycles.

Let’s see if you can get the actual codes before making more guesses.


2001 Volvo S40.

The code is P0420

Mileage is 103,000.

My key concern is that this code comes and goes. So, about 3/4 of the time, the computer is happy with the performance of the converter. The remainder it is complaining. I’m just reluctant to put $1,200 into a car that’s worth about 2,500 on trade in and 5,000 retail.


I’d look harder at the O2 sensors before even thinking about replacing the cat. I don’t think the problem is the cat.


This vehicle showed “bad” O2 sensors at around 50K miles. Taht still did not clear the issue. I ended up getting a PROM update at a Volvo dealer. The issue was that the tolerances were set very tight, and false codes were being set.

This is a case where I’d like to put the car on a dyno and give it a stress test. Older cars were given a dyno stress test. Now that I have a OBDII car, they just look for no codes. I’m not convinced that we know the issue.