New catalytic converter burns more gas

I recently had the catalytic converter replaced on my 1999 Toyota Corolla and the gas mileage dropped from about 28 mpg to 21 mpg. Any answers on why.

Couple of questions . . .

Why was the catalytic converter replaced?

What kind of cat was installed . . . direct fit, universal?

What condition is the engine in . . . as far as you know?

Were the oxygen sensors also replaced . . . front and rear, that is?

If so, what brand was installed?

Did you initially have a check engine light, which led to the cat replacement?

Is the check engine light still on?

it was replaced because the engine light came on. I tried running sea foam through it but the light kept coming back on. it had 150,000 miles on it so I figured it was time to replace it. I am not sure what kind of cat was used but my personal mechanic replaced with some type of generic brand, not one made by Toyota. I believe the cat cost around 250 plus 200 labor. engine runs good but burns a couple quarts of oil between changes. oxygen sensors were not replaced. the engine light is no longer on and it passed the emissions tests which is required here in Georgia.

“engine runs good but burns a couple of quarts of oil between changes.”

That excessive oil consumption is probably a major reason why the cat needed to be replaced

Don’t expect the replacement cat to last as long as the original part did

If it’s any consolation, older Toyota 4 cylinder engines . . . in the cars, at least . . . are well known for excessive oil consumption

thanks for the reply but any reasons for the drop in gas mileage and is there anything that can be done to correct it.

How many tankfuls have you checked the mileage on?

I suppose this is an aftermarket converter?

At least 5 to 6 tankfuls. I know it is not a Toyota made catalytic converter. My mechanic got it for 250 dollars.

Back pressure may be to blame for the drop in fuel economy. If the light is out count yourself lucky. I would drive it another few weeks and see if the fuel economy approves. I believe it will.

It’s been several months since it was replaced.

Not sure

Anyways . . . is the engine getting up to proper operating temperature?

Is it doing so in a timely manner, or is taking forever?

After the cat was replaced, did it seem like the car was down on power?

it seems to run like it always did.

There’s nothing about an aftermarket converter that would affect your gas mileage as long as the mechanic got a direct fit “OEM-replacement” part (NOT to be confused with an OEM part). If he got a generic converter and adapted it in, it may very well be the source of your problem. Unfortunately, the OEM converters have a very generic look to them, so it may be hard to tell by looking (see attached link).

That would be my first question. Did he or didn’t he?

Re: the oil usage, a “couple of quarts between oil changes” would suggest a quart every 2500 miles +/-. That is perfectly normal and acceptable oil usage. That would be perfectly acceptable for a new engine with 10,000 miles on it (albeit a bit higher than normal) and is excellent for a 16 year old engine.

I should also add that 150,000 miles seems for some reason to be about the normal lifespan of Toyota cat converters. And at they age they do tend to manifest their expiring life by repeatedly setting the CEL. Generally it’ll start only occasionally and become more frequent as time goes on. But to really get to the root of the CEL, it would really be helpful to know exactly what the codes were. They should be written on your copies of the shop orders.

I’m going to be a pessimist here, and suggest the replacement cat may indeed be the problem

I’m going to assume a super generic, one size fits all cat was welded in

Db, you may be right. The configuration of that cat allows the use of numerous very generic (read: cheap & fits everything) converters. It’s just a straight can, the honeycomb of which could be just about anything. I’m hoping the OP posts back so we can have that discussion.

@Joe Cool, At first you said 5 or 6 tankfuls, then tell us it has been several months. So the big question I have is, just how much do you drive this car? Both my wife and I run through about a tank a week in our daily drivers. This equates to about 200 miles a week or 1,000 a month, close to 11,000 a year. If your driving a tank a month, 200 miles a month or about 2500 miles a year, mostly short drives, the cat may not be burned in yet. You need to take it on an extended drive a few times, at least 45 mins to an hour, to burn in the cat. That may help.

The replacement cat could be the problem, or the O2 sensors could be to blame.
If I was replacing a cat, I would also replace the O2 sensors.

This sounds like the perfect excuse to replace the CAT with a test pipe. They’re pretty cheap, just a piece of straight pipe, usually flanged to fit your application, and will answer your question absolutely. No doubt you will get better mpg, maybe a little louder, and you can switch it out for your annual smog inspection if you have one. IMO however, the replacement of the CAT as you describe it could not have caused the mpg to drop by 25%, gotta be something else. Look around for all of the normal stuff, vacuum lines knocked off or fallen off, brakes dragging, ignition timing off, tire pressure down, or a combination of all of them. Good luck! Rocketman

Thanks for all the replies. I didn’t replace the O2 sensors so I may try that if it isn’t’ to pricey.