I've had check engine light on for a while. Got it coded @ autoshop & was told it was an 02 sensor -- no big deal. I've been running for a while no problem but have now noticed a drop in gas mileage.

I plan on cleaning MAF & replacing the O2 sensor, but how dow I tell which is bad? From reading posts I see that my '97 4.0L XLT has 3 sensors.

Thanks.

A scan with an OBD II scanner will tell you which one(s) is/are bad,

Good luck.....

You should replace all of them at once anyway.

Also...you can drive on a bad O2s for a while, but one day, your truck will just quit running as you're driving down the road, and not want to re-start. Then you'll know it's time to quit putting it off and replace them...

Tale a look at the following thread about the problems a bad O2 sensor caused.....http://www.explorerforum.com/forums/showthread.php?s=&threadid=34619&highlight=alarm+system+woos