Install modifications

Steps on the car itself, outside the harness. Every harness for this car inherits them.

• Re-route the pressure-sensor vacuum hose to the intake manifoldrequired

  Re-route the pressure sensor's (Mazda "EGR boost sensor", pin 3S) vacuum hose directly to the intake manifold, bypassing the EGR boost solenoid. The stock ECU multiplexes baro + MAP through that solenoid: with it off the sensor reads atmosphere, with it energized it reads manifold (2000 FSM P0107/P0108). After re-routing the sensor reads MAP only, so the standalone needs its own barometric source — its onboard baro sensor if fitted, or a separate baro sensor on a spare analog input. (Alternative, not recommended: keep the hose stock and have the ECU energize the solenoid permanently via a low-side output switched with ignition; this costs an output.)

  Why: With the solenoid off (its default) the sensor reads atmospheric pressure, not manifold pressure, so a standalone that uses this sensor as its MAP input would read the wrong value.

  Where: EGR boost solenoid and the pressure sensor's vacuum hose (sensor signal 3S).

  Source: 2000 Mazda MX-5 Miata factory service manual — engine self-diagnostics (DTC P0106/P0107/P0108)