Q: check engine light fails to come on. on 2004 Mercedes-Benz C320

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during emissions inspection the check engine light failed to come on. What are the possible causes and How can this be fixed.
(2) Answers
(2) Comments
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The ignition switch supplies power to the powertrain computer and the Check Engine Light on the dash, the light should come on when the ignition key is in the on position, if the bulb gets power (and the instrument cluster wiring board is OK) the computer grounds the bulb to turn it on, once the engine starts the computer removes ground to turn off the light and only grounds the wire if it then sees a fault. You would probably be bst locating the computer, and see if that ECU pin is grounding through the ECU and if grounding the pin makes the light work, otherwise you are looking at testing the circuit board or dash bulb.

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