oxygen sensor replace didnt fix idle

2001 windstar. Began idling rough and check engine light came on. Advance said engine code was for both downstream sensors. Replaced both oxygen sensors, check engine light is now off but car still very rough when idling. There is no code to check. What can cause this?

The Idle Air Control valve is a common source of Windstar idle problems.

Side note: The problem with oxygen sensors is that they're a potential cause of most any engine problem but, in my experience, rarely fail. So they're an easy suggestion for parts stores. End of side note...

Thankfully, the Windstar idle air control valve is easy to get to, and easy to service. In most cases, merely cleaning the valve is enough.

Here's a very nicely written page on cleaning the idle air control valve. Really, it almost takes longer to read about it than to do it:

http://leckemby.net/windstar/windstar02.html

Phil-l's suggestion could be right. IAC is a common problem I've seen reported here.

However, rough idle can come from a lot of source issues. For example, malfunctioning EGR (particularly stuck open) can also cause a rough idle. Usually you'd see a code for that, so without a code it's just a guess.

Another place to check is spark plug wires or plugs. Failed insulation or fouled plugs will cause the engine to run rough. Although it normally should have issues at any engine speed, I've seen spark problems that had stronger symptoms at idle speeds. However, I'd expect a misfire code to come along with a spark problem.

If you had access to a scan tool and some knowledge to use it, it can report helpful information about the operating status of the engine. For example, long term trim levels will inform you if PCM is adding extra fuel or taking fuel out to compensate for high/low oxygen levels in the exhaust. A vacuum leak can often cause idle problems. This shows up in the scan tool as high long term trim at idle, but normal readings higher RPM or driving conditions. Again, I'd expect a lean code to be reported for a vacuum leak.

This probably isn't very helpful, only to point out that without the code you're getting in to a shotgun approach to solving the idle problem.

One tip.....for ANY vehicle.
When you have a shop read the codes for you, ask them for the actual numerical code.
Few people ask for it....they just want to know what part to buy.
The actual code will help you determine the problem.....posting it in a forum, such as this one, will get you some help from folks who have had the same code/problem.

2001 is in the range that I see a LOT of coil pack issues. If you go to the work to get to it.....best to just replace it. There are 3 coils in the coil pack....each firing 2 cylinders. The 3 coils are encased in a nice molded epoxy block.
As a number of people have posted, mainly in the 1999 and newer windstars.....the epoxy body can crack. Then the high voltage can arc through this crack, causing the misfire.
Other things would be in the routine tune-up list for a vehicle of age....spark plug wires and spark plugs.
Make SURE to buy double platinum plugs....with the platinum enhancement on BOTH the center electrode AND the ground tab.
Stay FAR away from any of those fancy multiprong plugs.....they have been noted to cause misfire issues in the windstar.

Misfire can come from dirty fuel injectors.....a good fuel system cleaner helps.
A good STRONG cleaner is Berryman's B-13 Chemtool.

Another misfire....VERY common for windstar is clogged EGR ports.
This is a NORMAL issue, but the sad thing is that you have to remove the upper intake manifold to get to them and physically clean them out.
No cleaner in a can will do this.
EGR valve RARELY fails....however EGR ports being clogged is SUPER common.
Every so many miles (LOTS of miles....fortunately) the EGR ports need cleaning.

The EGR FLOW sensor is also a known issue.....called the DPFE.
The computer uses the feedback from this to know what the flow through the EGR line is.....and adjusts the EGR valve to get the "ideal" flow.
You can quickly see how a bad feedback signal can cause a misfire issue.

To see if the issue is anything to do with the EGR system, there is a test that you can do.
THIS WILL SET A CODE AND LIGHT THE CEL (check engine lite)
Unplug the vacuum line that connects to the EGR valve and plug it (a good clamp of a clothes pin). THIS will disable the EGR valve from opening (remember that the EGR valve RARELY fails in the Windstar).
Then go for a drive......if there is NO misfire issue, or rough idle (EGR valve is NOT supposed to open at idle).......then the EGR system or clogged EGR ports are NOT the cause of your problem.
Don't forget to re-connect the vacuum line to the EGR valve and to clear the CEL.

Again....go over the routine maintenance items FIRST.....I would hate to see you spend $$$$ on exotic things, just to find that it was something routine. Been there and done that.

Note that my pictures, that the link in my signature takes you to can give you a general idea of some things.....but mine was a 1996 and there were MAJOR changes made in 1999.

As for the IAC? YES....it is an issue. Interestingly.....I have to clean the one in my 2003 Toyota Sienna MUCH more often than I ever had to in my 1996 Ford Windstar......my Sienna one sticks a LOT worse and MUCH more often!!!!
The usual symptom is that the engine dies at idle.....you have to keep your foot on the accelerator a bit to keep the engine from dying.