The PCM (Powertrain Control Module) constantly looks at the information from various sensors and
other inputs and controls systems that affect vehicle performance and emissions. The PCM also
performs diagnostic tests on various parts of the system. The PCM can recognize operational
problems and alert the driver via the malfunction indicator lamp (MIL). When the PCM detects a
malfunction, the PCM stores a
diagnostic trouble code (DTC). The problem area is identified by the
particular DTC that is set. The control module supplies a buffered voltage to various sensors and
switches. The input and output devices in the PCM include analog to digital converters, signal buffers,
counters, and output drivers. The output drivers are electronic switches which complete a ground or
voltage circuit when turned on. Most PCM controlled components are operated via output drivers. The
PCM monitors these driver circuits for proper operation and, in most cases, can set a DTC
corresponding to the controlled device if a problem is detected.

The PCM is programmed with test routines that test the operation of the various systems the PCM
controls. Some tests monitor internal PCM functions. Many tests are run continuously. Other tests run
only under specific conditions, referred to as Conditions for Running the DTC. When the vehicle is
operating within the conditions for running a particular test, the PCM monitors certain parameters and
determines whether or not the values are within an expected range. The parameters and values
considered outside the range of normal operation are listed as Conditions for Setting the DTC. When
the Conditions for Setting the DTC occur, the PCM executes the Action Taken When the DTC Sets.
Some DTCs alert the driver via the MIL or a message. Other DTCs do not trigger a driver warning, but
are stored in memory. Refer to the Diagnostic Trouble Code (DTC) Types for a complete list of PCM
DTCs and the driver alerts the DTCs trigger. The PCM also saves data and input parameters when
most DTCs are set. This data is stored in the Freeze Frame/Failure Records.

DTCs are categorized by type. The DTC type is determined by the MIL operation and the manner in
which the fault data is stored when a particular DTC fails. In some cases there may be exceptions to
this structure. Therefore, it is important to read the Action Taken When the DTC Sets and the
Conditions for Clearing the MIL/DTC in the supporting text when diagnosing the system.

There are different types of DTCs and the action they take when set. Refer to Diagnostic Trouble
Code (DTC) Type Definitions for a description of the general characteristics of each DTC type.

The PCM is designed to maintain exhaust emission levels to Federal/California/other standards while
providing excellent driveability and fuel efficiency. Review the components and wiring diagrams in
order to determine which systems are controlled by the PCM. The PCM monitors numerous engine
and vehicle functions. The following are some of the functions that the PCM controls:

3.1 / 3.4 L v6 engine data sensor locations.

•  The engine fueling
•  The
ignition control (IC)
•  The knock sensor (KS) system
•  The
evaporative emissions (EVAP) system
•  The secondary air injection (AIR) system (if equipped)
•  The
exhaust gas recirculation (EGR) system
•  The automatic transmission functions
•  The generator
•  The A/C clutch control
•  The cooling fan control
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Engine Operation
2.2L  3100 / 3400 
3.5L  3800 
4200  4.3L Vortec

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3.4L Engine Computer (PCM) Description