The washing machine wont agitate?

First of all check if the motor is running or not when you turn on the motor. If the motor is not turning on then this problem is due to fault in the motor. But if the motor is getting on and running properly then most probably this problem relates to stuck up or cracked drain pump pulley. It needs to be checked but if the pump pulley is functioning properly then check the drain pump belt. If the drain pump belt is worn out or loose or cracked/broken then it needs to replace. But if the motor is not running then before replacing the motor check the lid switch.There are many possibilities for this problem, but the most common and regular problem seen is as follows:--
There is a lid switch in the washer. The washing machine will not start spinning till the lid is closed. In some cases it has been seen that lid is closed still the washer does not spin. in this cases the lid switch has to be checked. if the lid switch is faulty then the closed lid will not be detected by the washing machine, and it will not start spin cycle.
This will help. please keep updated for any further assistance if more help required.Thanks.

The following link explains step-by-step how to replace a lid switch on a Kenmore top loader:

http://www.fixya.com/support/r3557193-replacing_a_lid_switch

If you have any questions about this repair, please post back and let me know. I hope this helps you.

NOTE: Kenmore is manufactured by Whirlpool so these instructions will apply.

Okay...it took me a while to find this information and make some sense of it. The "50" indicates a motor stall. That means the RPS sensor (which is located near the main drive motor ) does not sense any movement of the main drive motor under the wash tub. This could be related to your next error code which is the "DU". This is an "Door Unlock" error. You will need to perform the following checks if washer fails to lock (or unlock).

1. Perform Manual: Door Switch and Manual: Door Lid Lock tests.
2. Remove the console to access the machine/motor control assembly. This is accomplished by loosening the three screws in the back of the console. With the screws loosened, the panel lifts from the back first. Use care when handling as there is a small connector that connects to the operator interface board.
3. Locate the machine.motor control board on the right hand side (as viewed from the front of the washer). Visually check that the P12 connector is inserted all the way into the machine/motor
control. (There are three connectors on the right hand side of the control board. They are labeled as follows: BROWN (P10), BLUE (P11), and ORANGE (P12)
If visual check passes, go to step 4. If visual check fails, reconnect P4 and repeat step 1.
4. Check the lid lock motor winding and switches by removing P12 from the machine/
motor control and checking the resistance values shown in the Lid Lock resistance table below:

Motor Winding - Check P12-2 to P12-3 - 26 ohms (Lock Switch locked and unlocked).
Lock Switch Home - Check P12-1 to P12-4 - 0 ohms (Unlocked) Open Circuit (Locked).
Lock Switch Lock - Check P12-1 to P12-7 - Open Circuit (Unlocked) 0 ohms (Locked).
Lock Switch Lid - Check P12-1 to P12-5 - Lid Open (Open Circuit) Lid Closed (0 ohms).
LID LOCK RESISTANCE
If resistance values are good, proceed to step 5. If the switch measurements ae incorrect for unlocked (or locked) conditions, a problem exists in the door lid lock. Replace the door lid lock.
5. If the Manual: Door Switch or Manual: Door Lid Lock in step 1 failed, then the machine/motor control has failed: Unplug washer or disconnect power. Replace machine/motor control assembly.

There should be a tech sheet inside your washer that you can also get this information from. I have referenced these steps from the tech sheet. To access, you can either lift the washer from the front and reach the inside the cabinet from under the washer, or lift the top panel by pushing in on the release tabs. This is accomplished by inserting a putty knife along the seam where the top meets the front about two inched in from each side. Push in while lifting to release the tabs. CAUTION: Make sure you hold on to the washer lid to prevent it from swinging open as you lift the top.

The tech sheet is located on the inside of the front panel of the washer between the wash tub and cabinet interior. I mention this as there are helpful drawings and more detailed explanations to assist you. This can help avoid confusion from me explaining these steps to you. Since you mentioned you had just recently replaced the lid lock, you may double check your work to make sure you have everything reconnected correctly. Barring you purchased a bad part...if the lid lock proves to be good, you may have a defective control board.

I do hope you find this information helpful. Let me know if you require further assistance.

If this is a Kenmore, Whirpool, Estate, or Roper top loader the following link explains how to replace a lid switch:

http://www.fixya.com/support/r385198-replacing_lid_switch

If this is not one of the above mentioned models, please post your make and model in order to be provided with the answers you need. For future reference: All washing machines are not alike. Posting comments under a generic category usually won't get quick solutions for you. Post as much pertinent information as possible as this helps us assist you. This is meant as constructive advice. I hope you find this information helpful. Let me know if you still require assistance.

PS If this IS one of the above listed models, you do not access the switch through the back of the washer. The link epxlains.