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CES is on track, if it runs smoothly it's most likely due to a fault in the grounding circuit.

To test this, conect a length of wire to the M terminal on the start switch that has the black/yellow wire connected to it. When you turn off the switch to stop the engine, immediately touch the other end of the wire to the negative post of the battery and if the engine stops right away each time, the fault is in that circuit, most likely the switch.