Types of Codes:
NOTE: Some people have problems distinguishing the different codes. Read this section carefully. There are drawings of the code formats at the end of this section.
Fast codes contain the fault information output in the normal slow codes but are output about 100 times faster. These are the first things output on a Key on Engine Off test.
Hard faults are problems that the computer has located right now. Examples are a sensor out of range or a broken wire (open circuit).
The separator pulse is a single pulse that indicates the END of hard fault codes and the BEGINNING of memory codes. It will show up as a code 10 on most digital testers.
Memory codes are problems that the computer has noticed in the past. If for example there was a loose wire to a solenoid that only lost contact while driving but was making contact while testing the system there would be no hard fault code. The code would show up IN MEMORY. The same would happen for a sensor that only went out of range occasionally. Memory codes come out after the separator pulse.
NOTE: The computer will erase the memory after a certain number of engine re-starts if the problem does not repeat itself. The number of re-starts varies from 20 to 80 depending on the year of the vehicle. The later models keep memory longer.
The engine ID in a running test is a series of pulses equal to one half the number of engine cylinders. A 4 cylinder engine ID is 2 pulses, a 6 cylinder ID is 3 pulses and an 8 cylinder ID is 4 pulses. A diesel ID is 5.
A "goose" code (also called a dynamic response test) is output during an engine running test. This is a single pulse to signal you to quickly move the throttle approximately 1/2 way down and release. Note: Not all engines give a "Goose" code.
Codes are output as a series of pulses. The following charts show the (approximate) timing of the various code pulses.
NOTE: Two digit codes are shown. Three digit codes have similar timing. It just takes a little practice to distinguish between two and three digit types.
NOTE: Vehicle should be fully warmed for all tests.
See figure below. Eec Iv vehicles have two connectors for self testing the system. They are located on the firewall or the left or right front fender. The large connector contains the self test output (STO) and ground (sig rtn). The small pigtail is the self test input (STI).
Key On Engine Running (Koer) Test:
Use First Code Output and retest after any repairs are made.
Cylinder Balance Test:
NOTE: This test is only available on Sequential Fuel Injection (SFI) Engines. Start an engine running test and press the accelerator lightly within 2 minutes after the last code is output. The PCM will cancel each cylinder in turn and measure the RPM drop. If any weak cylinders are noted their number will be output as a multiple of 10 (e.g. 30, 40). The output is the actual cylinder number, not the number in the firing order. NOTE: Will not always pinpoint bad injectors.
Engine running or engine off tests Hook up for a self test but do not hook up the self test trigger. Turn key to on. Hook up the trigger, wait 10 seconds and disconnect. Hook up trigger again . Tap suspected sensors (be careful if engine is running), wiggle the wiring harnesses etc. IF the PCM picks up a fault the self test output will pulse and a memory code will be stored (The value of this is questionable. I NEVER located a problem with it. - JT).
To erase the memory disconnect the self test trigger while the codes are being output. You COULD also disconnect the battery but then the PCM forgets some important running characteristics.
Output State Test:
Run the KOEO test all the way through and leave the test hooked up. Cycle throttle 3/4 open and closed while watching tester, light or voltmeter. Self test output will switch with every throttle activation. On some testers (like my old pocket testers) the light will only pulse every other push. But the outputs still switch EVERY time.