Scan Tool User Guide
67
13: Diagnostic States (GM)
For some older GM vehicle engine ECUs, the Diag-
nostic States function lets you place a vehicle in
special test modes (states). The Road Mode is
normally used for diagnostic testing and three addi-
tional modes are available to check codes, serve as
a backup fuel setting, or function as a service setting
as follows:
•
Road Mode
—
should be used whenever
possible to diagnose driveability problems.
This mode does not include a preset test idle
speed or additional advance.
•
Field Service Mode
—
grounds the vehicle
diagnostic link and triggers the check engine
indicator light to flash. From the number and
speed of the flashes, you can determine DTCs
and the current state of the Oxygen (O2)
sensor.
•
Back-Up Fuel Mode
(also called limp-in mode)
—
keeps a disabled vehicle running until it can
be repaired. This mode supplies injector pulses
to the fuel injectors and sets base timing if the
Programmable Read Only Memory (PROM)
chip fails. Vehicle service manuals refer to this
mode as “3.9 K Ohm State” or “Factory Test
State.” (You cannot view sensor or switch data
in this mode.)
•
ALCL Mode
—
automatically sets a test idle
speed and additional ignition advance if Road
Test data is unavailable or unreliable. Vehicle
service manuals refer to this mode as “10 K
Ohm State” or “Special State.”
1
Follow the instructions in
Test Startup and Vehicle
Connection
on
page 16
to display the Diagnostic
Menu screen.
2
Select
Diagnostic States
and press the
ENTER
key
to display the Diagnostic States screen (shown
below).
Figure 13.1: Diagnostic Menu Screen and Diagnostic States Screen
3
Select a
Mode
and press the
ENTER
key.
Figure 13.2: Diagnostic States Screens (Road Mode and Field
Service Mode)
Figure 13.3: Diagnostic States Screens (Backup Fuel Mode and
ALCL Mode)
4
Press the
ENTER
key to set the mode.
5
Perform the necessary diagnostic tests.
6
If you select Field Service Mode, do the following to
read the DTCs and the Oxygen (O2) sensor state:
•
To read
DTCs
, turn the vehicle key on (engine
off) and consult the vehicle service manual to
interpret the light flashes.
•
To read the
oxygen (O2) sensor state
, start
the engine and count how many times the
check engine light flashes per second (one or
two) and notice if the light is on longer than it is
off, or vice versa. Interpret the flashes as
follows: one flash per second indicates closed
loop and two flashes per second indicates open
loop; light on longer than off indicates rich
running engine; light off longer than on indi-
cates lean running engine; light on and off
equal times indicates ideal running engine
(14.7 to 1 air/fuel ratio).
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