Miata Engine Computer Self Diagnostics

By Glenn A. Stephens

(Information from Miata Factory Service Manual and Miata Magazine)

The Miata engine computer continuously monitors many of the parameters related to engine performance such as fuel, spark, air, rpm, etc. It uses this information to get the most performance and fuel economy possible from the engine. A nice side effect is that it can detect many problems your engine may be experiencing and it even has a way to tell you about them.

When an out-of-range condition is detected a fault code is recorded in the engine computer. To read the codes, first open the diagnosis connector that is positioned just on top of the driver's side shock absorber under the hood. Raise the lid on the small black box to reveal the legend for the connector.

To test your engine, first start it up and let it reach normal operating temperature. Then shut the engine off and proceed with work at the diagnosis connector. Using an unfolded paper clip, connect the GND and the TEN pins together. Return to the driver's seat and turn your ignition key to "ON" but do not start the engine.

Observe the "CHECK ENGINE" light in your instrument panel. It will flash a certain number of times to indicate a malfunction in your engine's control system. Count the number of flashes to indicate the malfunction code.

If the code is a two-digit number, the "tens" digit will be displayed first, then for a short 1.6-second period the light will be dark, then the "ones" digit will be displayed. This will repeat after a four-second dark pause. If there are multiple codes, they will each be separated by a four-second pause and may include two-digit codes, so pay attention.

Take as long as you need to record the codes and then refer to the chart below or section "F" of your factory service manual.

Note that the following codes are for '93 and older models. Beginning in '96, the Miata used OBD (On-board diagnostics) which requires a special scanner to read the codes.

FAULT CODES ('90-'93)

1	Ignition pulse (No Igf signal)
2	No Ne signal
3	No G signal
8	Airflow meter open or short circuit
9	Water thermistor open or short circuit
10	Intake air thermistor in airflow meter open or short circuit
12	Throttle position sensor open or short circuit
14	Atmospheric pressure sensor open or short circuit
15	Oxygen sensor <0.55 V 180 sec after 1500 rpm
17	Oxygen sensor unchanged after 1500 rpm
26	Solenoid valve (Evaporative canister purge)
34	ISC Valve

01 IGF signal
03 SGT signal
04 SGC signal
08 Mass Airflow sensor
09 Engine coolant temperature sensor
10 Intake air temperature sensor (mass airflow sensor)
12 Throttle position sensor
14 Barometric absolute pressure sensor
15 Heated oxygen sensor (inactivation)
16 EGR function sensor
17 Heated oxygen sensor (inversion)
25 PRC solenoid valve
26 Purge solenoid valve
28 EGR solenoid valve (vacuum)
29 EGR solenoid valve (vent)
34 ISC valve