Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Key On/Engine Off (KOEO) Self-Test
There are 4 codes displayed during this test. The first is a Fast code, which is not applicable in field testing. Fast codes are represented by slight needle deflections on analog VOM or flickering /LED on STAR tester. These are standard service codes being run at a rate too high for the testers to read. Fast codes are used by automatic equipment during manufacturing processes.
In order of display, the next 3 codes used for this test are the On Demand code, the Separator code, and the Continuous/Memory code. The first service code shown in this test sequence is the On Demand code. The On Demand code indicates a malfunction which is present at time of testing. This type of malfunction is called a "hard fault".
The second code is the Separator code, which indicates that On Demand codes have ended and Continuous/Memory codes are about to start. The third code is the Continuous/Memory code, which indicates a recent problem which is not present at time of test (intermittent fault).
The On Demand first digit is indicated by a pulse or pulses, after which the needle drops to zero for 2 seconds before the second digit of the code is indicated as a pulse or pulses. After a pause of 6-9 seconds, the Separator code (single digit on VOM; 10 on STAR tester) will be displayed for 1/2 second. After 6-9 seconds, the first digit of the Continuous/Memory code will be indicated, the needle then dropping to zero for 2 seconds, and then the second digit of the Continuous/Memory code will be indicated.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.