Quite a few things. EEC-IV has two main diagnostic tests, KOEO (key
on/engine off) and KOER (key on/engine running). EEC-IV also monitors
sensors and engine parameters during normal operation, and stores
trouble codes for any problems found (Continuous Memory).
KOEO goes through all the sensors, relays and actuators and makes sure
they operate as expected, and dumps trouble codes for any problems it
finds during the test (KOEO codes), as well as any Continuous Memory
codes. This includes things not quite serious enough to light CHECK ENGINE.
KOER will raise and lower the idle speed, check the timing and mixture
controls, check the TPS, and check the brake on/off and power steering
pressure switches (if your car has them). It will also dump any trouble
codes it finds after the tests finish.
EEC-IV also has an Output State Check mode that will let you turn the
actuators on and off using the accelerator pedal, and a "wiggle test"
mode that will monitor the sensors and throw a code if a wiring fault is
detected. Sequential fuel injection applications also have a cylinder
balance test that can tell you if you have a weak cylinder.
All of this can be accessed with just a code reader; no scan tools are
required unless you want to see the real-time serial data later EEC-IVs
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