Is the engine check light on? If not, is the bulb burned out (turn ignition to check lights and see).

If the check engine light is on it could be a variety of problems - some easy to fix and some not. You can take the car to Autozone or Pepboys and they will read the codes from the engine for free, then post them here for help.

My 1994 840 failed smog last spring and I didn't know the check engine light was burned out (otherwise it would have ben on). Turned out my EGR needed cleaning, and I did it myself following onlinbe instructions... with basic tools and little experience. The car ran better and passed smog no problem after I cleaned the EGR.

If the check engine light is working and not on, could just need a tune up or could be catalytic converter or something else outside engine.

Good luck!

When was your brick's last tune up?

I mean - plugs, air filter, clean flametrap/emissions valves, etc.

Was the car warm (fully warmed up - 10 miles 0n highway)) when it was submitted for testing?

What does your mechanic suggest?
--
'96 855R, 95 855, '95 854, 90 744TI 377,000 miles on 7 bricks

The most likely causes would be a leaking intake manifold gasket or possibly a bad spark plug coil. Does it idle rough or vibrate at idle? What are the co and nox readings at? to test a spark plug coil, unplug the wires at the coil packs while the car is running one at a time and see if you have a dead cylinder. if you have one not working at idle, swap it with another coil and see if the dead cylinder moves with the coil pack. if it moves the coil pack is bad, if it stays, the intake manifold gasket may be bad on the intake runner for that cylinder, thats a good starting point.