O2 Sensors Bad?
 

How do you know if your O2 sensors are bad. I'm getting horrible mileage, and have completed a full tune-up. Is there any way to check them? No check engine light, and no codes on the scanner. Any Help. Thanks.

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91 GT, T-5z, B&M Ripper, 3.27, 2 1/2 MAC cat-back, 3 core rad, 180 thermostat, KYB's, 245/50/16 all around

For one thing, how many miles do you have. My guess is that it should be change after about 60-70k miles but I still have mine at 90k. I'm planning on changing it soon because my Check Engine light blinks every once in a while when the engine is cold under moderate acceleration.

In your case, I think your engine is running too cold and never getting up to optimum operating temperature, therefore, causing it to run a little rich. Try changing your themostat back to a 190 degree one and then check your mileage.

Update of information. My Mustang has 137,000 on it. Changed the O2 Sensors back at 90,000. I've already put the 190 thermostat back in. It runs at operating temp. Tires have correct pressure. No ideas.....?....