Mazda Miata 1995-1997 ECU - Troubleshooting Guide

(Use this only if you’re still having issues after our ECU service.)

• Step 1 – Confirm ECU is powering up

  • Battery must be fully charged (12.4–12.7V KOEO).

  • Check all ECU/ENGINE/IGN fuses in under-hood and interior boxes.

  • Turn key ON and listen/feel for main relay and fuel-pump relay clicking.

  • If relays don’t click, recheck ignition switch power feed and grounds.

• Step 2 – If you still have U0100 / P0600 (no communication)

  • Verify scan tool is set to the correct year/engine and OBD connector is fully seated.

  • Check continuity from OBD-II pin 7 (K-line) to the ECU diagnostics pin.

  • Make sure no aftermarket alarm/remote-start is spliced into the ECU or OBD wiring.

  • Repair any damaged wiring, corroded connectors, or added devices and retest.

• Step 3 – If the engine cranks but will not start

  • Check for RPM signal on your scan tool while cranking; if 0, inspect crank/cam sensors and wiring.

  • Confirm you have spark at the plugs and injector pulse with a noid light or test light.

  • If no spark and no injector pulse but ECU powers up, focus on crank sensor, igniter/coil wiring, and engine grounds.

• Step 4 – If it starts but runs poorly / stalls or shows P0601–P0605 / P1600 again

  • Clear all codes, drive or idle the car until the fault returns, then re-scan.

  • Inspect all ECU grounds to the engine and chassis; clean and tighten if any are rusty or loose.

  • Check for low system voltage while running (alternator or main ground issues can cause “internal ECU” codes).

  • Verify no other modules or add-ons are sharing ECU power/ground in a way that could pull voltage down.

• Step 5 – When to look beyond the ECU

  • If powers/grounds, fuses, relays, sensors, and wiring all test good, remaining issues are likely in vehicle wiring, sensors, ignition components, or fuel system rather than the ECU.

  • At this point we recommend a full in-vehicle diagnosis with a professional technician who can scope signals and load-test circuits.