Many carbureted and all fuel injected vehicles covered by this manual have self-diagnostic capabilities. The ECU monitors all input and output functions within the electronic engine control system. If a malfunction is detected, the information will be stored in the ECU memory in the form of a 1 or 2-digit code. Single-digit codes (as used on all 1985 models, as well as many 1986–87 vehicles) can be accessed through the use of Mazda's System Checker 83 (No. 49 G030 920) or equivalent, which uses a red lamp and buzzer to indicate sensor or related wiring faults. Two-digit codes (as applicable to most 1986–89 vehicles) can be accessed through the use of Mazda's Digital Code Checker (No. 49 G018 9A0 or equivalent) or Mazda's Self-Diagnosis Checker (No. 49 H018 9A1 or equivalent). When using one of these testers, the codes will be output and displayed as numbers, such as 09. Regardless of which type of tester is used, the tester's wire connector must be attached to the vehicle's control unit check connector. Follow the tester's instructions for specific hook-up and testing procedures.
NOTE: If such a tester is not available, the codes may instead be read using an analog voltmeter. When using a voltmeter, the code's digits will be represented by needle sweeps.