Actron OBD II AutoScanner CP9135 Instructions Manual page 38

About the Malfunction Indicator Lamp (MIL)
All OBD II compliant vehicles have a "Malfunction Indicator Lamp" or MIL. In
the past, the MIL has been referred to as a "Check Engine" or "Service Engine
Soon" light.
Normal operation
The engine computer turns the MIL ON and OFF as needed. This dashboard
message is either amber or red, and maybe labeled with "Check Engine",
"Service Engine Soon", "Service Engine Now", or marked with a small engine
picture or diagram
The MIL is normally OFF when the engine is RUNNING.
NOTE: The MIL will turn on when the ignition key is in ON position, but
the engine is OFF prior to starting the vehicle. This is a normal
test of all the dashboard message lights.
Problem spotted
If the MIL does not come on during this test, you may have an electrical
problem which needs repair. Refer to the "Diagnostic Circuit Check" steps of
your vehicle service manual.
Current problem
When the MIL remains ON after the engine is RUNNING, the computer sees
a problem that does not go away (known as a "current" failure). The light will
stay on as long as the problem is present and a trouble code is stored in the
computer's memory. Use the OBD II AutoScanner at the earliest convenient
time to obtain codes.
Intermittent problem
When the MIL comes ON and then goes OFF while the engine is RUNNING,
the computer saw a problem, but the problem went away (known as an
"intermittent" failure). Though the MIL went OFF because the problem went
away, the code stays in memory. Use the OBD II AutoScanner at the earliest
convenient time to obtain codes.
NOTE: The computer will automatically erase these codes after
repeated restarts if the problem does not return.
Poorly running engine, no MIL
Most likely, this condition is not due to computer system failures, but reading
codes can still be useful as part of a basic troubleshooting procedure. Check
wiring and bulb for "Check Engine" light failures. Refer to vehicle service
manual for additional diagnostic information.
On OBD II vehicles, the MIL also signals an emissions-control related failure.
The vehicle may not run any differently, but the OBD II system is designed
to note very small changes in the engine's operation which could lead to
emissions damage or failure.
B-5
About OBD II