The AE86 ECU has some basic OBD1 diagnostic ability, and can
output codes to signal fault conditions on a number of the engines
sensor systems. The codes can be extracted by putting the ECU
into "Diagnostic Mode" and reading the flashing codes that are
displayed on the Check Engine light. A full list of the codes is
contained in the Factory Service Manual.

Fault codes are displayed when two pins (T and E1) on the
"Diagnostic Plug" are shorted, and the ignition switched ON.
The engine will go into diagnostic mode providing certain
conditions are met. These are...

1. Battery voltage 11 volts or better.
2. Throttle fully closed (TPS IDL switch closed).
3. Transmission in neutral.
4. All accessories switched OFF.
5. Engine at normal operating temperature.

One other use for the diagnostic plug is to put the running engine
into diagnostic mode for setting the base ignition timing. You will
know that the engine is in diagnostic mode if the engine speed
drops by about 100 rpm, and the timing drops back from 16-17 deg
(typically) to 10 deg base timing. If this does not happen, then the
engine has not gone into diagnostic mode.

Cheers... jondee86