How do I diagnose and fix a Check Engine Light Error 01 on a Mazda RX-7 FC?

Difficulty: Moderate

On an S5 FC RX-7 (1989-1991), a Check Engine Light Code 01 specifically indicates a trailing coil spark cut ¹. This means the ECU is commanding the trailing coil to fire, but it is not receiving the feedback signal confirming the spark actually happened.

If this is putting the car into limp mode and causing a massive loss of power that temporarily resets when you cycle the ignition, you are dealing with a classic FC electrical fault ⁵.

Here is how to diagnose and fix it:

1. Verify the Code If you haven't already, confirm you are pulling a Code 01 by grounding the green single-wire diagnostic plug located near the driver-side front strut tower and reading the CEL flashes ².

2. Test the Coil and Igniter The most obvious culprit is a dead trailing coil pack or its attached igniter. However, don't just throw parts at it—community experience shows that replacing the coils often fails to resolve a Code 01 ¹.

3. Inspect the Wiring Harness Generally on this platform, if the coil and igniter are good, the issue is a break in the feedback wire between the trailing igniter and the ECU. You will need to test for continuity between the igniter plug and the ECU harness. I don't have the exact pinout numbers for your specific ECU, so don't take a guess—refer to the Factory Service Manual (FSM) wiring diagrams for the exact pins. Check the harness carefully for brittle wires, backed-out pins, or corroded grounds.

4. Check the ECU and Relays If the coil, igniter, and wiring all test good, the ECU itself is likely failing, which is a known cause for a persistent Code 01 ¹.

Before replacing the ECU, check the ignition switch contacts and the main ECU power relay under the dash. These are common failure points for intermittent power loss and reset-on-restart symptoms ³.

Note: If you do need to replace the ECU, make sure you disconnect the battery first to prevent shorting any circuits.