Q: Engine light came on. Car drives slow and stopped going in reverse. Could it be a knock sensor or is it the transmission?

Hi there. If the vehicle is driving slow and will not go into reverse, then the computer could have the engine in limp mode due to either the transmission not shifting into reverse. If there was no signs of gear slippage, then the transmission shift sensor (switch) on the transmission may have became damaged or the shifter cable could have come loose. Check the shifter cable for any damage and if its loose. Check the transmission position sensor and see if the plug is hooked up all the way. If the sensor is hooked up properly and you see no damage, then the switch may have gotten moisture inside it causing it to not read reverse to the computer. Run a scan on the engine to check the codes so you will have a starting point of diagnosis. If you need further assistance with your engine loss of power and there is no reverse, then seek out a professional, such as one from Your Mechanic, to help you.