Initial Inspection

You should always begin with the Flow of Diagnosis before doing the Diagnostic Circuit Check. Refer to the Flow of Diagnosis under Testing and Inspection. Flow of Diagnosis

The on-board diagnostic system check is an organized approach to identifying a problem created by an Electronic Engine Control System malfunction. It must be the starting point for any driveability complaint diagnosis, because it directs the service technician to the next logical step in diagnosing the complaint. Understanding the chart and using it correctly will reduce diagnostic time and prevent the unnecessary replacement of good parts.

OBD System Check Diagnostic Chart:

ECM Power And Ground Circuit:

Test Description
Number(s) below refer to circled number(s) on the following diagnostic chart.
1. This check is to see if the Engine Control Module (ECM) can provide a ground path for the "Check Engine" malfunction indicator lamp, this is a bulb check. If it can do this, it confirms that the ECM has power, ground and is capable of some functions. If the malfunction indicator lamp is "OFF", Chart A-1 will check for both ignition feed and constant battery power to the ECM and the ECM grounds.

2. This check is done to see if the ECM has the capability of performing internal diagnostics. Diagnostic trouble code 12 means there is no crankshaft position coming to the ECM, this is normal because the engine is not running.

3. This check is used to see if the ECM can supply serial data for Tech 1 "Scan tool" use.

4. Look at all the parameters to determine if one of them is not in a normal state with just the ignition "ON" and engine "OFF". In particular, look at the barometric pressure reading from the MAP sensor voltage and kPa values. Are they normal for your altitude? If not, use Chart C1-D. Look at the Engine Coolant Temperature (ECT) Sensor value to see if the value is shifted above or below where it should be. If so, see diagnostic aid chart on Diagnostic Trouble Code 14 chart.

5. This test is used to see if the cause of a "No-Start" is because of an ECM problem or the vehicle electrical system.

6. This check is to see if the ECM has any diagnostic codes stored in it memory to aid in diagnosing the customer complaint.