39
MCU SYSTEM
Using the Code Scanner (MCU Systems)
Complete Description for Reading and Using Service Codes
Do This First
This section shows you how to use the
Code Scanner for:
• Running tests of the engine computer
system.
• Reading service codes to pinpoint
problem causes.
Before using this section:
— Read Sections 1 and 2 to learn about
service codes and the Code Scanner
tool.
— Read Section 3 to find the location of
the Self- Test connector in your
vehicle. The connector type will tell
you whether you have an MCU
system or an EEC-IV system.
— Read this section (7) if you have an
MCU system. Use Section 4 if you
have an EEC-IV system.
Self-Test Summary
The Self-Test procedure (also called
“Quick Test”) involves engine off and
engine running tests.
The entire procedure is summarized
below. Each part is fully explained on
the following pages. IMPORTANT: All
parts must be performed as shown for
accurate test results!
Self-Test Summary
Part 1: Test Preparation.
• Safety First! Follow all safety rules.
• Perform Visual Inspection. This often
reveals the problem.
• Prepare Vehicle. Check choke voltage
and warm-up engine.
Part 2: Do Key On Engine Off
(KOEO) Self-Test.
• Get service codes to help pinpoint
problems.
Part 3: Do Key On Engine
Running (KOER) Self-Test.
• Get more service codes to pinpoint
problems found during engine
operating conditions.
Self-Test Part 1:
Test Preparation
1) Safety First!
• Set the parking brake.
• Put shift lever in PARK (automatic
transmission) or NEUTRAL (manual
transmission).
• Block the drive wheels.
• Make sure ignition key is in OFF
position.
2) Perform Visual Inspection.
Doing a thorough visual and “hands-
on” underhood inspection before
starting any diagnostic procedure is
essential!! You can find the cause of
many drivability problems by just
looking, thereby saving yourself a lot
of time.
• Has the vehicle been serviced