Why engine check light glows all time

Most vehicles having EFI systems and other electronics have several indicators with separate error codes related to the indicator; check engine indicator for P type emissions errors/B for body codes/C for chassis codes/U for network codes, ABS error codes when the abs indicator remains on, and airbag indicator for airbag error codes. A reader can decode P type error codes thru the OBD II port under the steering wheel/driver's footwell area. OBD II ports are D type with 16 pins.