Had the vehicle in for service and they said they should be changed. I do not have an issue with them, no DTC or CEL. They have never been change since I have owned the truck. Is there a service interval or do I just wait until they go?
Would it help with fuel economy? Found a site were it said they should be change around every 100K but nothing in Ford service recommendations.

Here's a webpage I found:

REPLACING OXYGEN SENSORS FOR PREVENTIVE MAINTENANCE

PREVENTING DRIVEABILITY PROBLEMS
Many driveability problems can be prevented by simply replacing aging O2 sensors for preventive maintenance. Though some import vehicle manufacturers recommend replacing O2 sensors at specific mileage intervals, others do not. Even so, replacing the sensor as regular preventive maintenance can provide numerous benefits.

Bosch recommends replacing unheated one- or two-wire O2 sensors on 1976 through early 1990s applications every 30,000 to 50,000 miles. Heated three and four-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. And on 1996 and newer OBD II-equipped vehicles, the recommended replacement interval is 100,000 miles.

A good oxygen sensor is essential for good fuel economy, emissions and performance.

If a vehicle with a sluggish O2 sensor is only averaging 18 mpg, and is driven 12,000 miles a year, replacing the sensor can save $100 or more a year in fuel bills if the new sensor improves fuel economy 10 percent to 15 percent (which it often can). It’s not a big savings, but there are other benefits, too. As we said earlier, bad O2 sensors are a major cause of emission failures, as well as a leading cause of catalytic converter failures. Replacing an aging O2 sensor for preventive maintenance, therefore, is recommended not only to restore peak fuel efficiency and to minimize exhaust emissions, but to also prolong and protect the life of the converter, too.

 

What are you working on and what is the mileage?

There is no prescribed interval to change O2 sensors. A competent technician or even owner can look at the output of the sensors and see if they're sluggish or lazy. Without doing that check first, simply replacing them is preventative guesswork.

 

Assuming there's nothing wrong with the currently installed O2 sensor, who says a new O2 sensor is going to work better than the one installed? Changing a good working O2 sensor could create a problem that doesn't exist. In other words...if it ain't broke, don't fix it.

 

2001 Ford Windstar 3.8L with 193K it runs fine currently no CEL just the ABS & Brake light issues. I guess the Monroe service center was trying to get more work out of an alignment job. Went in for a $89.00 alignment left with a $1400 Quote. I was planning to see if my scanner live data setting can read the o2 sensor voltage and go from there. Do you guys know the Voltage numbers if my scanner reads it?

 

Upstream O2 sensors should cleanly switch between .1 and .9 volts when the mixture is at stoic.

Yeah, that's called "sucker 'em in the door with a headline price and service and then see how much we can up-sell them".