You can check engine codes by shorting terminals A&B on the ALCL (That's "Assembly Line Communications Link", in case you wondered.) connector that's behind the cigarette lighter panel. You'll need a #15 TORX driver to remove the panel and expose the diagnostic connector. The orientation of this connector may vary, but refer to the little bump or "key" on the connector to find out where A&B are.
On my car the key is down, so A and B are the two bottom contacts on the passenger side. If the key is up, A and B will be the two top connectors on the driver's side.
With the ignition off, take a bent paperclip or other handy piece of wire and short (connect) A and B.
Turn the ignition key to the "run" position but don't start, and the Service Engine Soon light will blink the codes, starting with the code 12 (three times) and continuing with the real trouble codes (each three times), followed by the code 12 again to signify "that's all, folks!"

The following table of codes and associated sensors or other failures is taken from my 1986 Helms manual. Other years' Fieros may have slightly different or added codes, especially the later years with DIS instead of regular ignition.

#	Interpretation
13	Oxygen Sensor Failure
14	Coolant Temperature Sensor Failure
15	Coolant Temperature Sensor Failure
21	Throttle Position Sensor Reads Too High at Closed Throttle
22	Throttle Position Sensor Reads Too Low at Closed Throttle
24	Vehicle Speed Sensor Failure
33	Manifold Air Pressure Sensor Reads Too High
34	Manifold Air Pressure Sensor Reads Too Low
35	Idle Air Control Failure (or Vacuum Leak??)
42	Electronic Spark Timing Failure
45	Rich Exhaust (Possible Oxygen Sensor Failure?)
51	ECM PROM Failure
55	Faulty ECM