Q: My tachometer gauge needle is pointing straight down, and it won't start.

Hi there. Check the crankshaft position sensor and see if there is power to the sensor. If there is power to the sensor, then put leads from a digital meter on the sensor and crank the engine over. Check to see if sensor if working with a voltage reference on and off. You can be getting voltage to the spark plugs, but it may not be enough. Also, check the three essensuals of the engine, air, spark, and fuel. The fuel pressure regulator could have failed causing the rail pressure to drop making the engine not start. Check the fuel rail for the pressure and see if there is a minimum of 35 psi. If the pressure is lower, then I recommend replacing the fuel pressure regulator. If you are still having trouble getting your engine started, then I recommend seeking out a professional, such as one from Your Mechanic, to help you.