Introduction

The ECM has an on board diagnostic system, which detects malfunctions related to engine sensors or actuators. The ECM also records various emission-related diagnostic information including:
- Diagnostic Trouble Code (DTC) Mode 3 of SAE J1979
- Freeze Frame data Mode 2 of SAE J1979
- System Readiness Test (SRT) code Mode 1 of SAE J1979
- 1st Trip Diagnostic Trouble Code (1st Trip DTC) Mode 7 of SAE J1979
- 1st Trip Freeze Frame data
- Test values and Test limits Mode 6 of SAE J1979

The malfunction indicator lamp (MIL) on the instrument panel lights up when the same malfunction is detected in two consecutive trips (Two trip detection logic), or when the ECM enters fail-safe mode.





The engine has an ECM to control major Systems such as fuel control, ignition control, idle air control system, etc. The ECM accepts input signals from sensors and instantly drives actuators. It is essential that both input and output signals are proper and stable. At the same time, it is important that there are no problems such as vacuum leaks, fouled spark plugs, or other problems with the engine.




It is much more difficult to diagnose a problem that occurs intermittently rather than continuously. Most intermittent problems are caused by poor electric connections or improper wiring. In this case, careful checking of suspected circuits may help prevent the replacement of good parts.




A visual check only may not find the cause of the problems. A road test with CONSULT (or GST) or a circuit tester connected should be performed. Follow the "Work Flow".
Before undertaking actual checks, take a few minutes to talk with a customer who approaches with a driveability complaint. The customer can supply good information about such problems, especially intermittent ones. Find out what symptoms are present and under what conditions they occur. A "Diagnostic Worksheet" like the example above should be used.

WHAT......................Vehicle & Engine Model
WHEN......................Date, Frequencies
WHERE....................Road Conditions
HOW........................Operating Conditions, Weather Conditions, Symptoms.

Start your diagnosis by looking for "conventional" problems first. This will help troubleshoot driveability problems on an electronically controlled engine vehicle.