O2 sensor multiple replacements, terrible gas efficiency

I drive a 2000 Hyundai Accent with 82,000 miles on it. The past two years I’ve changed the o2 sensor attached to the front of the engine 4-5 times. I’ve done this because the gas mileage would go down then my check engine light would come on. Changing out the sensor would fix the issue for awhile. Now my check engine light is on again and the code is for a malfunctioning catalytic converter. I once again am getting terrible gas mileage. But the information I’ve been given is that the cat converter wouldn’t effect the gas mileage that way. And I was told I have an “engine” issue. I’m not sure where to go from here. When I tell mechanics I’m getting bad gas mileage they all tell me the same thing, that I need to replace my spark plugs and air filter or replace my fuel filter. I replaced the plugs and wires about a year ago. The air filter gets replaced two to three times per year. I haven’t replaced the fuel filter.

The problem isn’t with the fuel filter. If you’ve had to replace the front O2 sensor numerious times something is damaging that O2 sensor. And one thing that can kill O2 sensors over time is coolant slowly leaking into the combustion chambers.

Have someone check if there’s a small coolant leak into one or more of the combustion chambers.


It’s possible, even liekly, that the root cause of the oxygen sensor damage has also damaged your cat converter. Both can get damaged by deposition of unwanteds traveling in the exhaust stream. Both require clean and uncoated surfaces to function.

Stated differently, the cat converter itself will not affect the gas mileage, but that which is damaging the oxygen sensor will…and may even have damaged the cat converter.

I agree with the shop that said you have an “engine issue” and I agree with Tester to start by getting a compression and a leakdown test.

Thanks to both of you. I will start with your suggestions.