Last Week on My Mac: The myth of liquid detection

Macs have developed their own mythology, and this week I unintentionally came across a myth that developed over a year ago. Like so many it was born from a chance observation, this time of a new background process that appeared on 25 October 2023 in macOS Sonoma 14.1, and was reported in 9to5Mac on 3 November 2023.

Discovery

The process in question is liquiddetectiond, and as 9to5Mac’s headline claimed, it meant that “Macs can now inform Apple if any liquids have been detected in the USB-C ports”. That article argued that “it seems more likely that the data collected by this daemon will be used for technicians to determine whether a Mac is eligible for free repair.” “Putting a digital liquid detector on USB-C ports is just another way to ensure that technicians are right about claiming that a Mac has been exposed to liquids.”

That, and a couple of linked reports elsewhere, brought a small flurry of comments about how typical this was of Apple, then all went quiet until 9to5Mac’s article was picked up by Hacker News on 9 January 2024 and generated 340 comments. Predictably, most either castigated Apple’s behaviour or disappeared down rabbit holes about unrelated topics. Among them, though, was one precious insight: “It prevents the device from applying/draining power from any pin in such a state, mainly to reduce corrosion of the contacts and increase longevity.”

By the middle of January last year, the story had gone cold, and everyone must have gone away with their worst fears confirmed. You couldn’t even get a USB-C port damp in your Mac any more, as Apple would use that as an excuse to void your Mac’s warranty.

Documentation

Apple’s first word on the subject seems to have been in a support note published on 23 November 2024, which passed largely unnoticed. This announced liquid detection as a feature new to macOS Sequoia when running on only the following models:

MacBook Air M3 and later
MacBook Pro with M3 Pro or Max
MacBook Pro with M4 base, Pro or Max

none of which been released at the time of 9to5Mac’s report, although the second were released four days later, on 7 November 2023.

If there’s liquid in one of their USB-C receptacles (ports) when a USB-C cable is connected to it, this new sensor should detect it and alert the user, advising them to shut the Mac down, disconnect all cables and leave it to dry.

This is in addition to, and separate from, what Apple terms Liquid Contact Indicators (LCI), that have long been fitted to laptop Macs and some Apple wired and wireless keyboards “to help determine if these products have been exposed to liquid,” according to this support note.

Was Apple just making excuses, or was this new liquiddetectiond service intended to benefit the user?

Evidence

I stumbled into this innocently last week when I was looking at Accessory Security, a feature confined to laptop Apple silicon models. By chance, the laptop I was using was a MacBook Pro M3 Pro, one of the few in which liquid detection works. There, on several occasions in its log, after connecting a Thunderbolt cable, its liquid detection system checked that the USB-C port was dry, in a series of log entries like:
0.887 liquiddetectiond Starting LDCM Now
0.887 liquiddetectiond LDCM Discovery is enabled.
0.889 liquiddetectiond LDCM – Matched with V4…
0.890 liquiddetectiond LDCM – checkIsReceptacleEmpty: 0
0.890 liquiddetectiond LDCM – Handling LDCM interrupt event for port 2
0.890 IOAccessoryManager IOPortFeatureLDCMUserClient::_copyData(): Copying LDCM data… (target: Port-USB-C@2/LDCM)
0.890 liquiddetectiond LDCM – Feature Status: 0, Completion Status: 0, Measurement Pin: 0 Mitigations Status: 0, Wet: 0, Wet State Duration: 0
0.890 liquiddetectiond LDCM – checkIsReceptacleEmpty: 0
0.890 liquiddetectiond LDCM: liquidDetected: 0, receptacleEmpty: 0, shouldShow: 0
(Times given in seconds elapsed.)

But on my more recent Mac mini M4 Pro running Sequoia, all I saw was that LDCM is not supported on this device.

Attempts to connect over the network are obvious in the log, and on not one of the occasions that liquid detection was performed did that MacBook Pro try to connect to any remote site. Maybe its reports could have been embedded in other analytics data passed to Apple later, but there was absolutely no evidence that the results of liquiddetectiond went beyond the confines of my Mac.

This demonstrates the importance of testing out hypotheses, and of reading the log. Even without the benefit of Apple’s recent support note, it should have been easy to demonstrate this behaviour, yet no one seems to have attempted to.

Explanation

Claims made of the role of liquid detection in USB-C ports also don’t make sense. As with most laptop manufacturers, Apple already builds Liquid Contact Indicators into components of laptop Macs within their case. These are most frequently affected by spillage of drinks on a laptop’s keyboard, resulting in any of a wide range of water-based liquids from coffee to cognac entering the case. That often results in extensive damage to the logic board and other components, that are expensive to replace.

But a damp USB-C port is quite a different matter. It could occur in a laptop that had been out in the cold and was then brought into a warm and more humid environment, the same sort of conditions that steam up your spectacles. Over time, that could lead to corrosion of the contacts in the USB-C ports, and unreliable connections.

Because each release of macOS is identical across all models of Mac, although only a few of the most recent models feature liquid detection sensors in their USB-C ports, the liquiddetectiond service runs in the background of all Macs running Sequoia. It’s to be found inside /System/Library/CoreServices/liquiddetectiond.app, which isn’t even a bundle, just its Mach-O binary and an image of the warning sign displayed. It’s run through its LaunchDaemon com.apple.liquiddetectiond.plist, which you’ll also find in the SSV of every Mac.

As is so often the case, the truth behind the myth is more prosaic, and doesn’t involve Apple secretly capturing data from your Mac, nor conspiring to dodge warranty repairs. In fact, if you look at the warranty terms of pretty well every other laptop manufacturer, they too exclude damage caused by liquid ingress, as demonstrated by their Liquid Contact Indicators. And some are also starting to fit similar liquid detection sensors in their USB-C ports. But don’t let those get in the way of a good myth.