[PowerPC603]

pawn Код:

COMMAND:engine(playerid, params[])
{
    new engine, lights, alarm, doors, bonnet, boot, objective;
    new vid = GetPlayerVehicleID(playerid);
    GetVehicleParamsEx(vid, engine, lights, alarm, doors, bonnet, boot, objective);

    if(engine == 1)
    {
        SetVehicleParamsEx(vid, 0, lights, alarm, doors, bonnet, boot, objective);
        SCM(playerid, COLOR_GREEN, "You have turned off your vehicles engine.");
    }
    else if(engine == 0)
    {
        SetVehicleParamsEx(vid, 1, lights, alarm, doors, bonnet, boot, objective);
        SCM(playerid, COLOR_GREEN, "You have turned on your vehicles engine.");
    }
    return 1;
}

Your variable "engine" was always 0 when you entered the command, as you didn't get the engine-status first by using GetVehicleParamsEx.