TL;DR: Software locked it out because front passenger side door latch detects a nonexistent key in its nonexistent keyhole. It ultimately wasn’t, but could have been, dependent on which hand you used to open it!
Before I begin – and before anyone pictures me with cape and golden locks blowing in a heroic breeze – it was the dealership1 who finally cracked it, but it’s a strange story nonetheless, and a cautionary tale about complexity and interconnected systems.
For the last several years I’ve been chasing an intermittent fault in my daily driver Mazda 3: Sometimes, at random, the trunk (liftgate) wouldn’t open. On this model, the liftgate is opened by pushing a rubberized button on the outside, which energizes the mechanism and it pops open with a satisfying clunk (except when it doesn’t).
Initially, the problem happened just often enough to be mildly irritating, but not enough to sink serious time or resources into fixing it. It got better in summer and worse in winter. It worked more often for me, and acted up more often if my wife drove it. When my travels took me to the dealership (the better part of an hour’s drive in traffic, each way) for unrelated reasons, it was never playing up, so they wouldn’t touch it.
“Worse in winter”, you say? Give it some fresh grease, and if that doesn’t work, you’ve been blessed with free debugging by a planet-sized can of freeze spray. Find and jiggle every connector on the wire until you find the iffy crimp. Right?
If it were that easy, I’d have no story to tell!
No amount of jiggling cables, slamming or “percussive maintenance” on the liftgate, or trying it in different positions made a difference, but a pattern was emerging. The trouble seemed to be connected to the lock/unlocked state of the vehicle; it wasn’t intermittent on a push-by-push basis, but on a drive-by-drive basis, and, very rarely, repeated locking-unlocking without moving the vehicle would get it working again, largely ruling out intermittents in the cabling. It doesn’t necessarily mean anything, but the non-working behavior happens to be identical to trying to operate the latch with the vehicle locked – pushing the button just doesn’t do anything. No struggling sounds or dimming lights of a stuck motor, no signs that it is trying.
Hmm. What if somehow the latch remembers the lock state of the vehicle, and it’s a mechanical doohickey inside that’s getting sticky? That would explain the sensitivity to cold and locking. That’s gotta be it, right? Eventually I needed a few other fixes best handled by the dealership, and convinced them to look into it, ultimately replacing the latch.
It worked! At least until I drove home and it abruptly started playing up again. Back to the dealer for some more extensive diagnostics. Over the next few days, a flurry of increasingly speculative updates. It could have been…
- a Grounding Issue(tm). This is a legit common cause of unexplained vehicular gremlins, and also the ultimate shoulder-shrug of a mechanic who doesn’t know where to look next, depending which side of the service counter you’re on. But after they had cleaned up some ground wiring, wasn’t that.
- “Are you left-handed?” I had ruled out most forms of stupid-smarts, like sensitivity to what gear the transmission is in or how many times you pressed the unlock button first, secret-firmware-update-behavior-change, etc. but this one was news to me. On this model, the liftgate latch (not the others) is specifically sensitive to a passive proximity transmitter (i.e. RFID chip) in the key fob. If the receive range for this is degrading, e.g. due to a Grounding Issue at the receiver end, intermittent failures could depend on whether you keep the key in your left vs. right pocket vs. which hand you open the door with, or your height, pants vs. shirt pocket, etc. And ambient temperature, of course.
What it actually turned out to be, in the mechanic’s words, was they caught the “computer” (I presume to mean via OBD scan) showing a mechanical key present in the front passenger door lock, which prevents open requests from the liftgate from being honored. This may be surprising, since while this model does have a physical metal key (hidden in the keyfob as a contingency for a dead fob or car battery), it only works in the driver’s side door – the others have no keyhole to put it in. But as it turns out, rather than design separate mechanisms, they share enough “guts” that the others are capable of reporting a phantom key in just the right failure conditions. So, to fix the trunk latch, replace a passenger door latch. Obvious, right? ;-)
- “Stealership” jokes aside, the local Mazda dealership tends to be cost-competitive on basic stuff like oil changes and battery replacement, and more importantly, has never given me the “need a shower afterward” feeling of the local Jiffy Lube. You know, trying to sell new brakes, rotors and a coolant flush at 15,000 miles, and “do you really want me to put back this filthy air filter?” (holds up air filter clearly from a different class of vehicle, caked with leaves and bugs that don’t grow in your climate). Extremely by-the-book cuts two ways; they’ve never tried to sell unnecessary work, but also won’t do cheapout fixes like quickly disconnect the cable on a wonky, but superfluous, touchscreen rather than replace the full assembly. ↩︎
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