Really Bad Gas Mileage

When my car was new it was getting 36 miles/Gallon. Now it gets 25. I think I’ve checked all of the recommended things but nobody knows why. One thing that has come to my concern is that the catalytic converter has been replaced 3 times in the past 12 months. The guys at AAMCO after replacing the cat for the 3rd time did some tests but said they found nothing. Anyone have any ideas?

Maybe try not using those guys on your next visit? I found a great local mechanic when I moved to my new state by asking the local car enthusiasts. Just my opinion (as uneducated as it is). Good luck!

My first suggestion is to stop taking your car to AAMCO!

Do not take it to Midas, Meineke, Monro, Sears, Pep Boys, or any other chain either.
These places are notorious for incorrect diagnoses and poor-quality, overpriced repairs.
Their mechanics also work on commission, so the more parts that they replace, the more they get paid.

Seek out an independent mechanic in your area for proper diagnosis of the problem.
If you have skipped any of the maintenance procedures listed in the Toyota Maintenance Schedule, have those things done first in order to get a proper diagnosis.

the switch to an ethanol/gasoline mixture also contributes to a lower mpg…have no reference on when you were getting 35…also change of weather, that and an
off-brand cat would lower gas mileage.

Please tell us the year of your Corolla, and the current mileage.

Here’s an idea: take the car to a real mechanic, instead of AAMCO.

There’s NO WAY a catalytic converter should need to be replaced three times in a year. This is proof that the people who are working on your car have no idea what they’re doing.

They probably installed a cat that isn’t correct for your car. Why are you letting a transmission place fix your exhaust system, and what was wrong with the original cat?

25 mpg in a Corolla is only acceptable if you live and drive in the heart of New York City. Otherwise, you should be getting 30+, as you used to.

AAMCO is never going to solve this. You have to cut your losses and go elsewhere. You’ll probably have to pay for a correct cat, but it will be worth it in the long run.

Stay away from national chain shops. They are nothing but bad news.

If the cat was replaced three times in the past year there’s definately something wrong with the way the engine is operating. And an engine that’s running rich will damage the cat. And the drop in fuel mileage indicates that the engine running rich.

One thing that can cause this is a thermostat that’s stuck partially open. If the thermostat is stuck partially open, the engine may never reach full operating temperature which will cause the engine to run rich. Another thing that can cause this is a faulty engine coolant temperature sensor for the engine computer. If this sensor it telling the computer that coolant never gets above 45 degrees it’ll cause the engine to run rich.

If these two items haven’t been check out that’s where I’d start.


“Why are you letting a transmission place fix your exhaust system”

Mcparadise–Of course I agree with you, but this person must have succumbed to AAMCO’s new advertising campaign, where they portray themselves as “the place for all your car care needs”–or some BS to that effect.

I guess that if a local AAMCO franchise has already ruined most of the transmissions in their area, then it is time to move on to new areas to screw up, but it is REALLY sad that people go to these charlatans. And, now they go to them for more than just the rebuilt transmission that every person with a trans problem is talked into!

Thanks for the advice about AAMCO. I had already learned my lesson after the first cat went bad but since I’ve already paid them to do it I’ve been taking it back to them to make them get the job right.

Last time I changed the thermostat was in California about 3 years ago. I remember I had to take off the alternator to get to it not a fun job.

I’ve never had the engine coolant temperature sensor checked either. I’ll defiantly have these 2 things checked out.

with the cat, I’d say maybe the O2 sensor(s) are faulty. Some vehicles will throw a code for the cat, but it is usually the o2 sensor that’s needing replaced

Tell them you have been advised that 3 cats. in a year means they are missing something.

I already tried explaining that to them last time. Their response was that sometimes cats just go bad. I didn’t buy it but what further could I have asked them to do at the time?

I had the O2 sensors and the cat replaced at 100K miles. I don’t think the sensors are the issue because if they were bad they would kick the light back on within a week. I then had the cat replaced at around 167k and AAMCO put on a nonstandard cat to save money. A few months later the light came back on again and they said the non standard cat would not work and I’d have to get the factory cat. Then 9 months later the cat was replaced again. They said they ran testing but could not find anything and that some cats just go bad. I don’t buy it! That’s why I’m asking here.

It’s a 2001 Toyota Corolla Sport with around 175K on it now.

“what further could I have asked them to do at the time?”

Give you a partial refund to help pay for a proper repair at a competent shop?

An oxygen sensor nearing the end of its life can drop you gas mileage long before it throws a code. How many miles are on your car ? Do you have the right converter for your car ? A one size fits all converter may be too restrictive for your car, leading to a richer mixture , dropping your gas mileage and burning out your converter. See the viscous cycle ?


I understand your frustration and I also understand that you want to have AAMCO make good on the repairs that they charged you for (or, perhaps that they repeatedly charged you for).
However, I believe that it is time for you to “cut your losses” and go to a competent independent mechanic, even if it seems right now that this course of action will cost you more money.

Consider this definition that is attributed to Albert Einstein:
Insanity: doing the same thing over and over again and expecting different results.

Please note that I am not suggesting that you are insane.
However, if you honestly believe that AAMCO is going to resolve this problem at no additional cost to you, then you might want to check the mental health coverage on your health insurance policy.

So I took it to an independent mechanic today which I found from this site. They ran a full diagnostic on the car and found nothing. I asked them about the thermostat and engine coolant level sensor but they said if those were an issue it would be apparent through engine performance, heater not working, or dash temperature gauge not reading correct. I’m not seeing issues with those 3 things. I asked them about friction so they put the car on the lift and told me the wheels were spinning fine. After leaving this place I went to the dealership to ask them. They said 4 things are most likely happening.

  1. When I brought the car from Colorado to Texas the gas just runs richer because I’m close to sea level.
  2. Texas requires 10% Ethanol in their gas. So far from what I’ve seen online Colorado does not have this requirement.
  3. The car has 175K Miles on it.
  4. Now that work is only 1.5 miles away and its city driving I’m seeing a drop.

As far as why 2 cats went bad within a year nobody seems to know.
What do you guys think?

“Now that work is only 1.5 miles away and its city driving I’m seeing a drop.”

Ummm…If you had bothered to share that VERY significant bit of information previously, we could have told you that this is the biggest factor in a drop in gas mileage.

Incidentally, you now need to change your oil under the Severe Service guidelines listed in your Toyota Maintenance Schedule. This means changing the oil on the basis of elapsed time, rather than on the basis of odometer mileage. More than likely, this will mean changing the oil every 3 months, regardless of odometer mileage. That won’t keep your exhaust system from rotting out, but at least it will help to preserve the life of the engine.

Even when I was living 20 miles away from work the gas mileage dropped to 29. I’ll keep that in mind about the oil though. I have some friends who are really preaching changing my oil every 1500 instead of the recommended 3000 anyways.

If the cat was replaced three times in the past year there’s definately something wrong with the way the engine is operating.

OR maybe it never needed on and AAMCO is just doing work as usual.