Diagnostic Aids

NOTE: After each service or repair procedure has been completed, reconnect all components. Clear DTCs and repeat QUICK TEST procedures to ensure all electronic engine control systems are working properly and DTCs are no longer present.

NOTE: CHECK ENGINE light is also known as Malfunction Indicator Light (MIL).

Perform this test when directed by QUICK TEST. This test is intended to diagnose the following:

CHECK ENGINE Light Circuit
Instrument Cluster
Powertrain Control Module (PCM)

Econoline & F650-750

CHECK ENGINE light is intended to alert driver of certain concerns in closed loop system. CHECK ENGINE light output is turned on when strategy detects a concern in certain input and output circuits. CHECK ENGINE light will remain on as long as concern is present. Regulations governing this light also require that DTCs be displayed by flashing of this light.

Fig 1: Identifying CHECK ENGINE Light Circuits (Econoline)
Courtesy of FORD MOTOR CO.

Fig 2: Identifying CHECK ENGINE Light Circuits (F650-750)
Courtesy of FORD MOTOR CO.

Excursion & F250-550

CHECK ENGINE light is used to indicate to operator when certain faults have been detected by PCM. When certain faults are detected, command to illuminate indicator is sent from PCM to instrument cluster through Standard Corporate Protocol (SCP) communications. With ignition on, indicator is commanded on as a prove-out function of indicator. CHECK ENGINE light is illuminated when strategy detects a concern in certain PCM input and output circuits. CHECK ENGINE light will remain on when a fault is present, and for OBD-II applications, CHECK ENGINE light follows guidelines for OBD-II fault detection.

Fig 3: Identifying CHECK ENGINE Light Circuits (Excursion & F250-550)
Courtesy of FORD MOTOR CO.

CHECK ENGINE Light Functionality

CHECK ENGINE light is located in instrument cluster and is Amber colored. CHECK ENGINE light functions in one of 3 modes:
1. 1) Bulb Check
  
  With ignition on light is turned on for a short duration as a bulb check. If this does not occur, check wiring, bulb, and associated fuse.
2. 2) Detection Of Certain Concerns
  
  CHECK ENGINE light is used to indicate that one of certain concerns has been detected by PCM or IDM. Only those concerns which result in a change in power output available from engine will cause light to be illuminated. Examples of such concerns are:
  - Injector Low Side Short To Ground
    
    PCM will turn off 4 cylinders to prevent engine damage.
  - Accelerator Pedal Sensor Out Of Range
    
    PCM will allow engine to idle only.