The ECM is equipped with a self diagnosis sytem which detects system failure and aids service by identifing the fault via a trouble code. The ECM uses sensors to look at many engine operating conditions. It has a memory and it knows what certain what certain sensor readings should be under certain conditions. If a sensor reading is not what the ECM thinks it should be, the ECM will turn on the SES (Service Engine Soon) light on the instrument panel, and will store a trouble code in the memory. The trouble code indicates which circuit the trouble is located in.
There are at least two ways to retrieve stored trouble codes. The first and easiest is with a scan tool. There are many manufacturers of scan tools with different models available. Although nearly all will attach to the ALDL (Assembly Line Diagnostic Connector), each operates differently and covering the proper procedures for those would be beyond the scope of this document. Please refer to the manufacturers operating instructions for the correct procedure.
Front view of ALDL connector.
The second method is to short terminals "A" and "B" of the ALDL with the key "ON", engine "OFF". This places the ECM into Diagnostic Mode. When the ECM is placed into Diagnostic Mode, the SES light will flash a code "12" indicating that the system is operating. A code "12" consists of one flash, followed by a short pause, then two flashes in quick succession. Code 12 will be flashed three times followed by any stored trouble codes. If no other codes are stored, code 12 will continue to flash until the diagnostic terminal is ungrounded.
|12||No Distributer Reference Signal|
|13||Open Oxygen Sensor Circuit|
|14||Coolant Sensor Circuit Shorted|
|15||Coolant Sensor Circuit Open|
|21||Throttle Position Sensor|
|22||Throttle Position Sensor|
|23||M/C Solenoid Voltage Low|
|24||Vechicle Speed Sensor|
|32||Baro Sensor Voltage Low|
|34||Differential Pressure Sensor||MAP Sensor Circuit|
|35||Idle Speed Control Circuit|
|41||No Distributer Reference Signal|
|42||EST or Bypass Fault|
|43||ESC Voltage Low|
|44||Lean Exhaust Indication|
|45||Rich Exhaust Indication|
|53||EGR Control Error|
|54||M/C Solenoid Voltage High|
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