I would try checking for codes using the diagnostic mode screw on the ECM:
Turn ignition On.
Turn diagnostic mode selector on ECU fully clockwise and wait until inspection lamps flash. Number of flashes displayed indicates corresponding mode.
Note number of flashes, then immediately turn diagnostic mode selector fully counterclockwise.
If ignition is turned Off during diagnosis, in each mode, and then turned back on again after power to the ECU has dropped off completely, diagnosis will automatically return to Mode I.
For description of modes, proceed as follows:
(Mode I): with engine stopped, system in bulb check state. with engine running, system in malfunction warning state.
(Mode II): with engine stopped, system in self diagnostic results state. with engine running, system in front O2S monitor state.