On pre-1996 US Miatas (OBD-I), you read trouble codes with no scan tool: jumper the TEN and GND terminals in the underhood diagnostic box, turn the key to ON, and count the CHECK ENGINE light flashes. The box is the black "DIAGNOSIS" connector near the driver's-side shock tower, with a legend inside its lid.
Procedure.
- Warm the engine to normal operating temperature, then shut it off.
- Jumper TEN to GND in the diagnostic box using a paper clip or jumper wire. Match the labels on the lid; do not guess.
- Key to ON, engine not running, and watch the CHECK ENGINE lamp on the dash.
- Read the flashes. For a two-digit code the tens digit flashes first, a short pause, then the ones digit; a longer pause separates one code from the next. Steady, evenly spaced single flashes mean no stored faults (system OK).
- No CHECK ENGINE lamp on your dash? You can read the codes on an LED wired between B+ and the FEN terminal (use a series resistor around 470 ohm) while TEN and GND are jumpered.
Write down every code before clearing. Match each count against the code legend for your year. Clear stored codes by disconnecting the battery briefly after the repair, then drive and re-scan to confirm the fault does not return. Because there is no live data stream on OBD-I, blink codes plus a multimeter at the sensor are your primary tools.