21
Three Way Catalytic Converter
ENGINE CONTROL MODULE
To obtain maximum effect from the catalyst and
accurate control of the air fuel ratio, the emission
certified engine is equipped with an onboard
computer or Engine Control Module (ECM). The
ECM is a controller which receives input data
from sensors mounted to the engine and fuel sys-
tem and then outputs various signals to control
engine operation.
One specific function of the controller is to main-
tain a closed loop fuel control which is
accomplished by use of the Heated Exhaust Gas
Oxygen Sensors (HEGOs) mounted in the ex-
haust system. The HEGO sensors send a
voltage signal to the ECM which then changes
the amount of fuel being delivered from the injec-
tors to the engine.
Engine Control Module (ECM)
The ECM also performs diagnostic functions on
the fuel system and notifies the operator of en-
gine malfunctions by turning on a Malfunction
Indicator Light (MIL) mounted in the dash. Mal-
functions in the system are identified by a
Diagnostic Trouble Code (DTC) number. In addi-
tion to notifying the operator of the malfunction in
the system, the controller also stores the informa-
tion about the malfunction in its memory. A
technician can than utilize a computerized diag-
nostic scan tool to retrieve the stored diagnostic
code and by using the diagnostic charts in this
manual to determine the cause of the malfunc-
tion. In the event a technician does not have the
computerized diagnostic tool.
HEATED EXHAUST GAS OXYGEN SENSOR
The Heated Exhaust Gas Oxygen (HEGO) Sen-
sor is mounted in the exhaust system to measure
the amount of oxygen present in the exhaust
stream. The ECM continuously monitors the
HEGO measurement to determine whether the
fuel air ratio is too rich, too lean and richen or
lean the mixture of fuel delivered to the engine. If
the ECM determines that a rich or lean condition
is present for an extended period of time which
cannot be corrected, the ECM will set a diagnos-
tic code and turn on the MIL light in the dash.
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