Until earlier this year, my gas mileage was steady in my 2003 Honda Accord EX. I could drive about 387 miles to the tank in town and close to 500 miles to the tank highway. The mileage did not differ significantly during different seasons of the year. When the check engine light came on in March of this year, I took the car to the dealership. With their device, the mechanics concluded that I had a code 420, which meant that my catalytic converter needed to be replaced. I paid for that to be replaced. I noticed immediately after the new catalytic converter that my gas mileage had decreased significantly. Within a few weeks, the check engine light came on again and this time it was the O2 sensor. I paid for that to be replaced. Still, I noticed the mileage was terrible. I took the car back to the dealership, and the mechanic started listing off the various things that could cause the drastic change in gas mileage (and by drastic, I mean 285 miles to the tank in town and 400 miles to the tank highway). We replaced my spark plugs, checked the air filter, checked the tire pressure, replaced a cracked air hose, checked another air intake thing on the right side of the engine. All of these things did not alter the poor gas mileage. I cannot help but observe that the gas mileage problem did not begin until immediately following the replacement of my catalytic converter. The mechanic assures me that there is not a connection there. His latest answer was that the ethanol in our gas can alter my gas mileage. He explained how this works, and it makes sense to me. However, all of the gas places that I have been going to for years have the "contains 10% ethanol" label, so I don't think that all of the sudden this would be a problem for my car. Will someone please give me some helpful feedback?

 

After you changed the catalytic converter there is sudden dip in the gas mileage.I think its because of it.I would suggest you to go to another mechanic and get it checked.

 

This dealership seems a bit odd to me... they seem quick to discount the cat, even though that being improperly installed/it being bad could easily be a cause of this problem (though I would think their would be another code if that were so).

Of course it could also be something else they damaged/disconnected/knocked loose. Are there anymore codes? What's the mileage on the car? Was there any other work done around the time you noticed the change besides the cat?

As brian67 suggested, if you have another mechanic you can trust, I would use them to get a second opinion.