What the hay

[Flooringit]

So I put on a 4 in stainless exhaust. Why I was there I decided to fix the leaky trans the gasket between extension and trans was bad so I pulled and fixed every seal man that was pricy. So I got it all back together no leak. Yeaaaaah how ever get in and blower motor didn't work. Then I drive 2 miles to get some trans fluid. And the od light was flashing. Craaap that happened to me once it was the shift fork pads making the transfer case jump out of gear. So I put on new ones. And all was good. But now I can't figure out why it's flashing the speedo works so I figure the vss is working. I didn't take the sensor out when I took off extension I wonder if I nicked the sensor putting it in. What sensor is that. I know it reads on the gear that the parking paw is on. I had to put that back on (the parking paw) it came off when I took off extension. But it worked. There's no trans fluid in there. So what does it do?