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I have had this trouble in a car. You must keep the chassis and the drive train grounds connected.

The ground from the engine, the one that returns the battery for starting and the ground from the computer must be at the same point. There is generally a wire that joins the two. It will run from the battery to a solid ground point on the chassis.

It is important to have this ground as many sensors, think oil pressure, only have one wire. The sensor is simply a pressure activated resistor. As the resistance changes, the voltage to ground changes. The computer senses this and reacts accordingly.

If the ground from the chassis to the engine is missing, then the engine is a floating ground and the computer will measure the voltage across the sensor plus what ever voltage drop there is between the floating ground of the engine and the chassis.

If the computer is sensing a bad reading caused by the floating ground, low oil pressure, low oil, etc, the computer will not let the engine start.

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