1997 Sable Bas Mileage

Hi,
I have a 1997 Merc Sable 3.0 vulcan with 160K miles. Lately the gas mileage has dropped. I have checked tire pressure, replaced the wires, plugs, pcv, air filter, ran injector cleaner, and sea foamed the intake, but still low mileage (compared to normal). No SES light present.
What else can I do?
Will the O2 sensors trip the SES light? That is the only other think I can think to make the mileage be so bad. I also have a fuel filter to change, but not sure if that would be the cause.
Any help would be appreciated.
Thanks.

If you have an Autozone, Advance parts or similar chain auto parts store near you, go in and ask them to check your codes. They'll do it for free. (They want to sell you parts, and they can't if you don't know what you need.) If there are no codes, or the parts store isn't sure what they mean, then come back here with that information and maybe you can get an answer that will help you.

Recently had the codes checked, before changing the wires and plugs, because the light came on and flashed for a short time. The only problem found was misfire cylinder #1. No other codes at that time.
I will take back and have them re-check.
Thanks.

Had the codes checked again, no codes.
Any other ideas?