I was asked to have a look at an early 2000's GM key fob that had gone through the wash. It wasn't working but previous experience with key fobs, had shown them to be pretty resilient to such things. I was confident that I could probably get it working again.
Step 1: Taking Apart
This particular style of key fob is very easy to take apart, with no screws involved. Just take a knife blade and gently pry apart the two plastic halves of the shell of the key fob. The two halves will come apart as shown in the picture.
Step 2: Gently Pry the Battery Out of It's Holder With a Blade
Gently insert a knife blade under the battery and pry it out of it's holder. Note how the battery is mounted. There is a negative and a positive. In the photograph you are seeing the negative side facing up. The other side is clearly marked as positive and has the name of the manufacturer on it. Before you put the fob back together, bend the metal finger a little bit so it touches the battery with more force. If you have a meter, check to make sure that the battery is over three volts. My measurement was 3.15 volts. You can also replace the battery if you want to.
Step 3: Take Apart the Other Side
Take apart the other side of the fob separating the three parts, holder, rubber button assembly and circuit board. At this time, bend up slightly the little metal fingers on the circuit board that make contact with the battery.
Step 4: Wipe Down Every Surface
Wipe down every part and surface with Methyl Hydrate or Isopropyl Alcohol. This should get rid of any residual detergent which could interfere with the operation of the device.
Step 5: Before Reassembly Allow the Parts to Dry for a Few Hours
Be sure to allow the parts to dry for a few hours before reassembly. This is especially important for the carbon touch pads.
Step 6: Reassemble Key Fob in Reverse Order
Reassemble key fob in reverse order taking care to make sure the positive terminal of the battery is facing the plastic case.
Step 7: Put Everything Back Together
Put everything back together and test if you have the car nearby. Unfortunately I didn't and I had to test using the next strategy.
Step 8: If You Don't Have the Car Nearby You Can Test With a Spectrum Analyzer.
I didn't have the car nearby to test the key fob so I needed to test for a signal. I looked up the FCC number for that key fob and got the frequency it broadcasts on. I set up the spectrum analyzer for approximately 315 MHz as the center frequency and put a small antenna on the input. I held the key fob a few inches away from the antenna and pressed one of the buttons of the key fob. The center of the display shows a strong carrier with two sidebands typical of the type of modulation used by most remote key controls of that generation. Knowing that there is a strong carrier signal on the right frequency and seeing that a signal is being encoded on it, there is a high probability that the unit is working.