'87-95 EEC-IV DLC and CEL wiring diagram with test switch

Self-Test Switch for '87-95 EEC-IV vehicles.
IF THE IMAGE IS TOO SMALL, click it.

Not all lighted switches are suitable for this mod. Note that the switch shown is technically a DPST with a shared common terminal, and one pole (terminal #1) wired to an incandescent bulb. Since these switches are prewired for terminal #1 to be ground and #3 hot (opposite of this circuit), an LED switch won't work. Also, some of these switches don't open the circuit to the light, so it would be on with the key. If this exact style of switch can't be found, either the hot wire can be omitted, or an UNlit switch can be used, or a DPXT switch can be wired to an external light.

When the switch is ON, it closes the circuit from STI (W/P) to ground, causing the EEC to enter its self-test mode (KOEO or KOER) and flash codes on the stock CEL/MIL in the dash. The switch's lamp won't flash codes - it merely indicates that the switch is ON. For the switch's lamp to flash with the CEL, a true DPXT switch would be required, and an LED switch could be used.

For more info on reading codes, see:


For jumper wire use, see:
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Owner caption January 2012 · Photo 4193 of 5000