I say do a oil change and see. If you have tampered with the Thermostat in the car, will also cause the OD not to engage. The vehicle has to come to proper operating temperature before the OD will start to work. If the heat does not reach normal midway mark you will not get OD. Also it could be a faulty switch. It wont be a serious issue as all other gears work. Also when you check the oil level make sure u start it and then put it in neutral and then check. Good luck.