Can having a defective cat result in bad gas mileage?
Having a cat at all reduces gas mileage.
and further to that, how essential is it to replace a defective cat? Can it ruin your car?
Legally, you have to replace it.
These days with all the computer garbage on a car it can cause damage using a test pipe.
A cat can be ‘defective’ in different ways, either clogged or poisoned. Different results.
If you are getting poor mileage something else is very wrong with your car.
As steve mentioned, convertors fail in different ways. If it fails in the clogging mode, it can very well reduce your gas mileage, but that won’t be the only symptom-- your car will also run very poorly.
As for if you have to replace it, if you ask the government, not to metion mother nature, the answer is yes, especially if you live in a state that does emissions testing. However, a defective catalytic convertor is not going to cause any long-term damage to anything else on your car, though some cars won’t run right without the correct backpressure from the convertor.
If your cat is reducing your mileage, can reduce mileage, but it would also likely cause reduced power at highway speeds. Trying to pass someone on the highway would make this very noticeable.
If it is defective, you really should replace it for a number of reasons.
[b] Why do you think it is defective? [/b] If it is due to a CEL, get the codes read and post the results back here. Just because you get an error codes does not always indicate the part referenced is defective.
It’s the other way 'round. Having bad gas mileage can result in a defective cat. Bad gas mileage comes from a poorly maintained engine and engine management system (engine computer, sensors, controls).