Diagnostic Procedure

Ensure all engine systems not related to Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all problems not related to DEC system have been repaired. DEC system diagnosis should always begin with DIAGNOSTIC SYSTEM CHECK chart under DIAGNOSTIC CHARTS in TESTS W/CODES - DIESEL article in this section. Performing this check determines whether DEC system and ECM are working properly.
If trouble codes (other than Code 12) are displayed, determine whether codes are hard or intermittent trouble codes. A hard code remains until code-setting condition is repaired. Hard codes cause Malfunction Indicator Light (MIL) to come on.
An intermittent code does not reset itself and is not present when vehicle is repaired. An intermittent code is often caused by loose connections. MIL goes out 10 seconds after fault goes away.

Fig 1: Diesel Fuel Injection Diagnostic Circuit Schematic (A/T)

Courtesy of GENERAL MOTORS CORP.

Fig 2: Diesel Fuel Injection Diagnostic Circuit Schematic (M/T)

Courtesy of GENERAL MOTORS CORP.

Fig 3: Diagnostic Circuit Check & Work Flow (Diesel)