Q: have replaced crankshaft sensor twice in one week? what can be causing this?

In your circumstance, there are a few failure modes that could potentially apply: If the crankshaft has a heavily worn main thrust bearing, the crankshaft will move around and the thin metal plate running through the crank sensor can repeatedly strike one side of the sensor with each engine revolution. You might hear a clicking noise. The effect is to wear down the crank sensor or break it completely. Another cause is excessive heat that can cause the plastic casing of the crank sensor causing it to melt or crack. If any of the teeth, or the pins, on the "wheel" are broken, snapped off, or worn down, that will disrupt the pattern of the pulses. The magnet can also attract metal particles and if those stick to the sensor or the wheel, they will throw off the sensor’s measurements. You might also have a fault in the wiring which is somehow temporarily masked each time you install a "new" sensor. The only way to conclusively tell what is going on is to confirm the actual failure mode of the sensor (exact physical or electrical cause of failure). Once the exact failure mode is determined - and a mechanic could do it for you, of course - the cause then has to be eliminated. If you desire a diagnostic, please make a request using YourMechanic’s electrical diagnostic page and the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.