- Where to Start

All Digital Electronic Fuel Injection (DEFI / DFI) system diagnosis should begin with the DFI System Check diagnosis chart. Testing and Inspection
After a portion of the DFI system diagnosis or the entire diagnosis procedure has been performed, only then should the ECM system portion trouble code diagnosis be conducted (only if otherwise directed by the actual DFI system check or diagnosis procedure charts should ECM portion diagnosis be conducted first). The DEFI system check (or DFI as indicated within the check and/or diagnosis charts) verifies the system is functioning correctly. Diagnosis must begin at this point. Short cutting these procedures may lead to improper diagnosis and wasted diagnostic time and/or result in the replacement of a component(s) that may be operating properly. It is important to note that on some engines the oxygen sensor will cool off after only a short period of operation at idle. This will place the system into the OPEN LOOP mode of operation. To restore the CLOSED LOOP mode of operation, start and operate engine at part throttle application for several minutes and cycle engine speed from idle to part throttle a few times.
The DEFI portion of the system diagnosis procedure is an organized approach for identifying a problem caused by the DEFI system. Driver comments normally fall into one of the following areas; Steady "Service Now" or "Service Soon" light, driveability problem or engine will not start or stalls after engine start-up.
The ECM grounds the telltales when the engine is cranking. If the telltales come on when cranking, then the ECM is getting power and ground. If the "Service Now" or "Service Soon" lights are on when the engine is running, check for stored ECM codes. In this case, diagnosis the codes FIRST before proceeding with the DEFI system check. After the codes are corrected, the ability of the DEFI system to control the fuel delivery to the engine must be checked.
The DEFI system check also checks for proper closed/open loop operation. If closed loop is achieved, the fuel system has to be checked for rich, lean or proper fuel control. The DEFI system has ECM parameter (controlling function) P.1.4 "integrator" which monitors the oxygen sensor rich/lean status and drives the fuel system richer if the oxygen is lean and leaner if the oxygen sensor is rich.