The full story, and sordid , arcane details..
These test below presumes you passed the Stall test, and the bark test. (and that the EGR exhaust flow paths are clean)
In all years, the ECU activates the EGR VSV (in front of motor) directly, off IDLE up to and never at W.O.T.
The TPS idle switch must NOT be closed and be at 0 volts. or the test (called a Monitor) will fail. or suspend.
In the early year's, you can just gun the throttle, in neutral(park) and see the EGR open.
Starting in the year 1994 they added, that the ECU reads the VSS sensor in the speedo ,and looks for greater than > 1 mph speed. (rear drive shaft (prop) speed , actual)
That is , the rear wheels must spin ! (per the FSM pages showing one crazy guy spinning a tire, by hand, in neutral.)
The test works in gear, or in park or neutral. Doing it in park or neutral is a safe way to check live action.
If you have hard basic failures this test can fail, like bad TPS, ETC, VSS1 , sensors?
IF the , VSS is dead you get DTC warnings, (P0500)
Keep in mind, the parked live test could pass, but fails only moving, due to other things failing, (say the PRNDL switch showing wrong gears)
The ECU must not be in limphome mode either, or the EGR tests are suspended. (retarded timing, very rich fueling, etc)
One guy found his EGR only activates for 1 second, with a dead VSS (a missing ground to VSS clicker switch, can happen at the speedo head.)
I'd say all other DTCs must be fixed first, unless the EGR leaks, (clean it , close it and disconnect the vac, hose, (plug it) and see if all other DTC errors are clear first. If not fix those first.
Vacuum leaks to the main plenum or running lean can cause false P0400.
The VSS is JUST a spinning magnet and a cheap 20 cent reed switch. nothing shockingly high tech here. It only needs to click to get the EGR ACTIVE.
The ECU activates the EGR main with the one simple VSV valve. (0 volts across its coil is OPEN valve, 12vdc closes the valve at idle)
When the ECU boots up, key on, the valve is closed with 12vdc.
The flow looks like this. The smog test is down in the middle there some where. The EGR flow must be there , or NOX SMOG tests, fail big time.
Vertical axis is EGR exhaust flow, and , the horizontal is engine load. The more load the stronger the PUTT.!
The EGR modulator senses this PUTT (#4 bark) and modulates vaccuum to get the below , effect.
If the CAT (or guts blown out) is missing , you get no PUTT or way too little EGR action.
The EGR must be routed correctly with vacuum holes. This below photo is the 1992-95 16v system. Federal. The 96 year they added a check valve.
Below, see the P hose this MUST go to the vented port (see top hat?) of the VSV1 or vacuum can be trapped. In EGR main. Causing it to stick.
The VSV top hat is a tiny air filter.
Five tests: (the EGR opens for as long as rear tires spin ,not WOT and not idle, if not ? then ECU has bad inputs.) {vss, dead?}
Not using a scan too to make sure all sensors work, is making this harder. P0500 is not good, fix that first or code 26.
Test 1: (The Live test fails , the next step is......)
I will assume the EGR main fails to open with the live test or only does so, for about1 second and then closes , during the live test.
The EGR main needs only 8 inches of vacuum to open fully to get the above flow profile.
The Throttle body has a PORTED EGR nipple, this goes to the EGR VSV1 in the front of the engine. See the 96+ vacuum pathes here.
Take a Tee fitting and attach it in line, to the center hose above. (cause no leaks and attach a cheap vacuum gauge, monitoring this line)
On a hot motor the vacuum here zero at idle.
You then open the throttle by hand and the vacuum exceeds, 5-8" Hg. Just be sure you have vacuum here the value is not critical.
This nipple on a 16v (top) must not be clogged, or the tiny EGR port, inside. (blocked at idle it is) The 8v TBI has a simular port.
Test 2:
I pull the fat hose off the bottom of the MOD VALVE idling it must do this PUTT PUTT thing, this is actual exhaust port #4 pulsing.
Just feel the hose with your fingers. for the pulses? its like less than 1psi pulses very weak.
(it's not very strong this putting , and the mod valve is very sensitive to this pulse. and come from here.)
I can blow on the E tube and close the valve with very gentle blowing. (to give you a scale of this pressure) At full engine load it gets way stronger.
test 2 passes, I'm doing the tests from easy to harder.
Test 3:
I move that vacuum Tee fitting and meter to, the above P vacuum line (sure I use a test hose). (P for plenum side or Pressure)
I then do that crazy real wheel spin trick in the page 4 of FSM or my (back one page).
The rear wheel spin trick ,is only for 1996 and newer cars. On the old cars, just race the engine in neutral.
The VSV opens (at zero volts) and the vacuum here, is 5 or just more Hg, not zero. and holds above zero, while over 1 MPH and advancing throttle, say 2000 RPM hot.
test 3 passes.
Test 4:
I move my Tee fitting and meter to the Q vacuum line and see the vacuum rise the faster I drive. (it may flutter ,or wiggle )
The test is using the same live test in test 3. (the range of vacuum is NOT zero up to 8" Hg. (vac. may go higher but the main is maxed out.)
I think the more load on the engine the more EGR flow, and vacuum to this test fitting. Drag the brakes as you apply throttle and get more vacuum.
The more vacuum the the more flow as the first graph above shows.
test 4 passes.
Test 5: if you look at the EGR main diaphragm, (its exposed) you see it move in farther under load.
The live test is now good. (that is, if the EGR complex exhaust path, is clear)
The 1996+ check valve VSV2 can fail and stick open and make the EGR dead. The VSV is tested like this.
This valve must stay closed.
FYI :The other vsv valves work like this.
This checks the whole electric and vacuum side.
Here is how the Mod valve works
The below pulse (PUTT) at red letter A, powers diaphragm B in the top right photo above.
The more load on engine ,as some point diaphragm B valve stays closed and full vacuum hits Nipple Q and main EGR .
v2