A common style of tub drain is the plunger type. It has a switch on the front of the tub, usually near the faucet, to regulate the flow of water down the drain. This is a mechanical switch with internal linkage which can get out of alignment and need adjusting, can rust and break, or can come apart at the joint. To fix a stuck tub drain switch, you will apply much the same technique as you would in replacing it. You may find that it's best to replace the mechanism rather than fixing it, so it's not a bad idea to buy one and have it on hand. You will not know to what extent your drain switch is damaged until you have removed it completely.
Step 1 - Prepare the Work Area
Cover the drain with some newspaper or a towel so that if you drop a screw, there's no chance of it going down the drain. Locate the drain switch and determine if you need a Phillips or a flat screwdriver. Some switches have a decorative metal plate over the screws and this will need to be removed before you continue.
Step 2 - Unscrew and Remove
Remove the two screws that are holding the plate in place and set them aside. Slowly lift up and move the cover plate back and forth at first, as it may be stuck. Slowly lift it out. Attached to the cover plate should be the linkage and at the bottom is a small plug. Remove this assembly completely.
Check for problems including rust and missing parts.
Step 3 - Possible Problems and Remedies
Dirty or Rusty - If the mechanism appears very dirty or rusty it should be replaced. If it is only a little dirty or rusty, you may be able to salvage it. Using a small wire brush, you should clean the linkage and plunger with some white vinegar. Use some heatproof grease and lubricate the assembly.
Continue to Step 4.
Broken - If there are missing parts on the assembly it has either rusted through and broken off, or it has simply come apart. You will need to fish out the remaining parts before you can continue. With a bit of patience, you can use a bent wire coat hanger to retrieve the parts.
If the unit has broken, you will need to replace if completely with a new drain switch. If the unit is simply detached, check it over to see if there is a visible cause. You can reassemble it and replace it, following step 4, or you can discard it and replace it with a new one.
Step 4 - Reassemble the Switch
Since this switch is part of the overflow system, protecting the tub from overflowing in the event of the faucet being left on and the drain closed, be careful that you seat it correctly. Feed the unit back in the opening the way you took it out. Feel around as you lower it and be sure the plug fits back in property. Screw the two screws back into the metal plate and tighten down firmly, but do not over-tighten. Replace any decorative plate you removed.