So I accidentally put my Honda Civic's car key through the washing machine. Oops. I tried unlocking the car as soon as I unloaded the machine and it still worked, phew. I didn't want to disassemble the key because I don't have a spare, so I just left it.
The following day however, I unlocked the car and loaded some things and it then unlocked again. Not great. The following day it happened more often, and I could see the LED on the key flashing each time, so obviously it was not working.
The key case has a stand-alone module inside, presumably sold by Valeo to various manufacturers to integrate with a Valeo immobiliser in the car. I removed the back of this case and could see moisture, dammit. I wasn't sure if the module needs a battery connected to 'keep' the coding, so I reassembled it without the silicone key cover, so it could dry out, however even though it looked dry the LED was randomly flashing.
A friend then suggested that detergent salts might do damage if left to dry naturally, so I then inspected it under my microscope and could see some deposits around the unlock button and around the main IC, so I squeezed some IPA onto the PCB and gently scrubbed it with a small brush, and then rinsed it with some more IPA. This seemed to remove most of it ok, although there were a few marks left.
Upon drying out the IPA - good news! It hasn't randomly unlocked since. I'm not sure if it was salts around the 'unlock' button switch or the IC, but either way the circuit must be quite sensitive to shorts.
One important thing in reassembling the module is that you need to put the PCB into the silicone cover first (it fits into a groove), then put them together into the clear front casing, and then finally snap the rear on which seals around the rim.
The silicone and rear cover clips do seem to get slack with use so I don't think that rim seals properly on older keys. If you have keys you regularly risk getting wet I'd recommend buying an immobiliser module off eBay (~£8) and swap your PCB into it. I recently got one and it looks brand new, it was probably from a spare key.
