Do you have an older chain drive garage door opener, and the motor no longer automatically stops when the door is at the fully open or closed position, keeps running until the timer or thermal cutout stops it?

Before you go to the extreme of replacing the opener or buying parts, try cleaning the switch contacts. In about an hour you can have your opener stopping on a dime.

Things you'll need

Multimeter with continuity setting
Small pliers
Thin putty knife or iSesame tool
Isopropyl alcohol
Electrical contact cleaner (I prefer Deoxit)
Emery cloth or fine grit sandpaper
Dielectric grease (optional)
Heavy duty pipe cleaner (optional)

Photos shown are from an Allister (Allstar) chain drive opener and the same process can be used for other similar units with mechanical limit switches.

Step 1: The Problem With Garage Door Limit Switches

A chain drive garage door usually has two small switches on the top of the unit, these are triggered when small rollers attached to the chain ride into the mechanism.

If your opener seems to "overrun" the switch and just keeps trying to pull the garage door up or down after it reaches the end of travel, you might assume that it "missed" the switch or was moving too fast. Before you go looking to bend metal or replace parts, you'll want to first check if the problem is as simple as a corroded switch contact.

Step 2: Remove Power

After closing your garage door partway, unplug the Garage Door Opener.

Verify that the opener does not have power. The light should be out, and the multimeter should show zero volts AC across any exposed electrical contacts.

Step 3: Identify Components

Your chain drive garage door opener may not appear exactly as shown, what you are looking for are the two limit switches, outlined in green and blue in this diagram.

Step 4: Check Continuity

Use your multimeter to verify that the switch is operating correctly.

Take your pliers, and carefully remove the two spade terminals from the top switch, exposing the contacts on the switch itself.
Set your multimeter to continuity (or resistance), and connect it to the two indicated terminals on the switch. It should show no connection (no continuity, or full resistance)
Push on the switch button (or push the spring arm to push the button), and the meter should show a connection between the two probes. Release the switch, and the connection should open.
Repeat the test with the contacts on the second switch

If the test fails, the terminals may be too corroded to make a good connection, or the switch may have internal problems. Either way, proceed to the next step to repair the switch.

Step 5: Remove the Switches

Using your pliers, carefully pull the terminals loose from the switch contacts, making sure you do not overly stretch the wires attached to each terminal. Make a note of which terminal goes to which switch contact.

Using pliers or a small socket wrench, remove the nut holding the faulty switch to the garage door opener body

Remove the switch. If both switches are faulty, repeat the process for the second switch

(The switches are identical, so don't worry about remembering which switch went where)

Step 6: Clean the Switch Terminals, and Then Re-test for Continuity

Wipe down the switch body and all three terminals with isopropyl alcohol
Using your emery cloth or metal polish, scrub the flat parts of each terminal blade until it is shiny
Apply electrical contact cleaner to each terminal, let sit for 1 minute, then wipe off
Repeat continuity test. If the test succeeds, skip the next step

Step 7: Surgery and Deep Cleaning

If cleaning the terminals does not restore continuity, you'll need to open the patient up

Your switch may be held closed by two melted plastic pins, these will need to be cut. Then you can use your putty knife or other flat tool to carefully pry the top cover off from the switch body.

Carefully clean all contacts as shown. Avoid using abrasives like emery cloth, as this will remove any protective plating and the switch will fail again soon

If you had to cut the pins to open the switch, you may need to hold the switch closed with a small cable tie after reassembly.

Once reassembled, repeat the continuity test.

Step 8: Reinstall the Switches and Test

Before reinstalling the switches, use your isopropyl alcohol and electrical contact cleaner to clean the terminals on the garage door opener.

Once the terminals are clean, apply a small amount of dielectric grease to both the switch contacts and the inside of the terminals, then slide the terminals over the contact. If the terminal feels loose, use your pliers to lightly squeeze them down to make good contact, but do not smash them flat -- you want to be able to remove these again in the future

Reinstall the switches over the screws, and tighten down the nuts to firmly hold the switch in place

Connect power and test the garage door opener