code reader with out check engine light on?

My 95 Aurora is running really really bad. At about 40-45mph and 1500rpms there is a hessitation/vibration type thing happening (like your bottoming out a gear on a manual trans) if I accelerate or decelerate it goes away...... also happens at around 70mph. Also idleing rough, gas mileage way down (like 13mpg), lost its ZOOOOM factor. Not a happy car! or owner for that matter.
Cars got 74,000 miles on it, did plugs and wires two years ago at 53,000, last week did fuel and air filter. This week had a friend bring it to work (car auction) for the mechanics to put it on a diagnostic computer and it was showing no codes! Mechanic said it would not show a code unless the check engine light was on (the check engine light is not on). Mechanic claims what it needs is a "good" tune-up.
Is the mechanic correct about the codes and the engine light?
Any ideas about the really really badly running car?

The mechanic is correct about the engine light and the codes. If there is no light there probably won't be any codes in the computer. The car may be running badly for the following reasons. First, make sure that you gapped the plugs correctly, the more miles on the plugs the more critical the gap. Then change the O2 sensors, they could be wearing out and causing the fuel mileage to go . Next, check the catalytic convertor. You are close to the mileage where the cat could be starting to plug or simply wear out. This should be covered under the emissions warranty of the vehicle. Finally, the problem you are having with at 40 to 45 mph could be the torque convertor solenoid going bad. Have it checked out at a tranny shop. You may also want to change the tranny fluid and filter. Don't have it flushed however, just a fluid and filter change.

if you have no codes and the engine does not run properly then check for fuel and vacuum issues.
Fuel and vacuum issues will usually not set a code unless severe.

check fuel pressure regulator first - at the end of the fuel rail; remove the vac line and see if it has any trace of gas - if it does replace it. a bad FPR can dump a-lots-a-fuel directly into the intake, far too much for the PCM to compensate giving bad mileage, rough idle, long crank times to start the car and inconsistant part throttle opperation.

check all the baisics first before going off on components.

air filter replace as required
fuel filter (replace)
all vac lines (especially PCV valve and hoses) for leaks
replace PCV valve
check intake bolts for tightness

a possible combination of problems takes me to a sticky EGR valve as a possible for the rough idle.

once all the basics are verified then start looking into components. however, realize that a component that is out of range will set a code and that's what stears me toward something less complex.

a clogged cat will usually not cause rough running but "clogged performance". the engine will run smooth but it will feel like something is holding you back because the exhaust gas cannot escape.

Thanx for the replies.
The Aurora is zoooomin' again........... it was a bad coil pack. I think this was the easiest and cheapest fix yet for this car.