If you failed NOX, you need a cat. Plain and simple.

IF you failed other readings, or they're high, a couple things should be done:

1) Tuneup- always make sure plugs, cap, rotor, and coil wire are ok.

2) intake vacuum leaks. Fix any hoses or the intake hose from AMM to throttle body. Replace the intake manifold gasket?

3) Air filter

4) Fuel pressure test. Costs about the same as a new fuel pressure regulator. Your choice.

There is a trick- the timing can be retarded to 0° on pre-89 cars. This reduces emissions drastically on most cars. You will be down on power. I wouldn't bring it up to the test official. They wouldn't like anything that you're going to undo after the test is complete.
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Rob Bareiss, New London CT ::: 87 244DL, 88 245DL, 90 745GL, 84 242DL project, 89 244 parts, 88 244DL to replace the 87