My 2002 did the same thing this year.  The air valves are mostly vacuum
powered.  If none of them work, like mine, then it is likely the vacuum
supply line is leaking.  I runs from the driver's side of the engine,
across the top (mine was pinched at a angle where it passed under some
other hoses), and proceeds to a vacuum manifold (a little thing) to the
curb side of center, accessible with the hood raised.  Unfortunately, to
replace it, I had to remove the engine cover inside to get at the intake
manifold end of the hose.  I just used standard rubber vacuum hose from
the parts store, rather than getting a replacement for the plastic hose
with rubber fittings that was installed at the factory.  Hopefully it
will last at least a few years in the heat.

I just fixed the same problem on my 2000 Astro van.

There is a plastic vacuum line that runs across the top of the engine,
toward the front. This line terminates at a small manifold on the
passenger side before going through the firewall behind the dash.

I was able to 'splice' this plastic line, which had broken right behind
the alternator, with a small piece of rubber vacuum line that was laying
around.

I did not even need to remove the doghouse for this repair. It was all
reachable under the hood.