What problems does a bad speed sensor cause

You replaced what speed sensor ? Do you mean the VSS - vehicle speed sensor ? Wheel speed sensors for the ABS - anti-lock brakes system are sometimes used for the speedometer info on some vehicle makes , but not yours . Where on the vehicle did you replace the speed sensor ? Could be the speedometer is bad .

indicates bad wires or connections
you may need a new sensor as transmission oil can get into the workings and cause problems

Hard to tell without knowing what year, make, and model of vehicle we are working on here. A speed sensor would not cause this, but bad engine or trans mounts could, or possibly a sticking valve body in the transmission.

The reason is the engine speed and transmission speed sensors don't agree. It could be caused by a bad transmission speed sensor, bad transmission fluid, a failed solenoid in the valve body ( in the transmission pan), a worn out valve body, a bad transmission, improper tires(that don't match each other), a faulty MAP sensor (air flow sensor on the air intake), faulty engine speed sensor and possibly a few other things.

Get the codes with a reader, and it might direct you in the right direction. But take the reading with a grain of salt, because it rarely tells you the exact problem. My fail safe came on and the code was TCC solenoid, but after replacing that, I had to install a rebuilt valve body to solve the problem.

The two are unrelated. A knock sensor code can mean the timing is off. Can also be caused by the ECU running lean, sometimes because of a O2 sensor. May be a dirty air filter. Can also be caused by bad gas or the wrong octane grade, or the cam belt has slipped. There's usually other error codes as well. The effect is poor gas milage, lack of power. It MIGHT cause other engine problems. Have all the above checked.

The blower motor failure is usually the switch in the dash. A common fault is that only some or one speed works.

It's very possible that your speed sensor is the root cause of all or most of your stated problems. The speed sensor is involved in many of the operations of the electronic engine control system. It would be worth the effort to replace the speed sensor with a new one, reset your EEC computer, and see what improves!

Yes.

From what I have read this sounds like a problem with the second speed sensor located on back at the rear axel on top of the differential. If the cable is loose or the sensor is bad it will cause the brake light to come on and the speedometer to act funny. There are two speed sensors...one in the transmission and the one on the differential. The second one is probably the problem.

For starters you need to find out which wheel is causing the fault. secondly some wheel speed sensors are built in to the wheel hub assembly, to identify what style you have depending if its in the front or rear.
look behind the wheel where the CV axle goes into the hub , the wire for the speed sensor will either go into a black fitting with 2 torx screws OR go directly into the hub. If the wire goes directly into the hub you will need to replace the entire Wheel Hub Assembly. if the wire goes into a fitting style you can just replace the sensor itself.


Hope this helps.

A faulty VSS (Vehicle Speed Sensor) or related wiring can cause both of your problems, trans and ABS. If the computer doesn't know the vehicle is moving, then it won't shift the trans. If the ABS controller sees wheel speed, but not vehicle speed, it will turn on the ABS light. The VSS is located in the rear transfer case housing. But remember, it may be the VSS, related wiring, a faulty speedo buffer (usually located in the dashboard), ECM, or even the reluctor on the transfer case output shaft. You may want to have a shop scan "data stream", or even have them diagnose the problem.