FAQ
TL;DR: On Mazda 3 BL that unlocks but won’t lock, “90%” of cases are GEM not seeing all latches; “You must think like the thoughts of the vehicle electronics.” [Elektroda, Anonymous, post #16699807]
Why it matters: This FAQ helps Mazda 3 BL owners fix a remote-lock failure that opens but won’t close, saving time and parts swaps.
Quick Facts
- Symptom set: remote opens; lock command gives one indicator flash and no action. [Elektroda, nikus1984, post #16687632]
- Mechanical key locks only the operated door during the fault. [Elektroda, nikus1984, post #16687632]
- Suspect list raised: lock-controller relay for closing circuit. [Elektroda, marek216, post #16691352]
- Alternative path: driver’s door lock or broken wires in the door grommet. [Elektroda, Ireneo, post #16694554]
- Edge case: third brake light ground can disrupt hatch latch sensing, blocking lock. [Elektroda, Anonymous, post #16699807]
What are the tell‑tale symptoms of this Mazda 3 BL locking fault?
The car unlocks from the remote, but on lock the indicators flash once and nothing locks. Using the key, only the door you turn locks. This pattern points away from fob battery issues and toward control or sensing faults. It matches the original case description. [Elektroda, nikus1984, post #16687632]
What’s the most likely root cause according to the thread?
A contributor states that in 90% of such cases the GEM module doesn’t detect all closures. If any latch isn’t seen as latched, the module inhibits locking. "You must think like the thoughts of the vehicle electronics." [Elektroda, Anonymous, post #16699807]
Could a failed relay be responsible?
Yes. One diagnosis points to the lock controller’s relay that drives the closing action. If the unlock relay works but the close relay fails, you get asymmetric behavior: opens fine, won’t close. Test the close-drive output before replacing parts. [Elektroda, marek216, post #16691352]
Should I suspect the driver’s door lock or wiring first?
Yes. Another expert suggests the driver’s door lock or a broken wire in the door grommet. Inspect the harness for fractured conductors, then test the lock’s response off‑car to rule the mechanism in or out. [Elektroda, Ireneo, post #16694554]
Why does a stop‑lamp error matter for central locking?
The third brake light in the tailgate shares ground with the hatch latch sensor. A poor ground can make the GEM think the hatch isn’t latched, so it refuses to lock. Fix the ground, and the system restores normal locking. [Elektroda, Anonymous, post #16699807]
How can I verify what the GEM thinks before replacing parts?
Use a scan tool and read the GEM’s live data for all door and hatch states. Confirm each shows "closed". Trigger the lock command and watch whether the module changes outputs. This prevents blind swaps and speeds diagnosis. [Elektroda, Anonymous, post #16701550]
Quick 3‑step how‑to: check GEM latch states with a tester
- Connect your tester and enter the GEM/BCM data stream.
- Close each door and the hatch; verify each input reads "closed."
- Press lock and observe outputs; investigate any input that isn’t "closed."
[Elektroda, Anonymous, post #16701550]
If I turn the key in the driver’s door, should all doors lock?
In the reported fault, turning the key only locks that single door. This asymmetry reinforces a control‑path or input‑validation problem, not a dead fob or general power issue. [Elektroda, nikus1984, post #16687632]
Does the immobilizer (PATS) have anything to do with this?
No. Participants dismiss PATS as unrelated to the central locking behavior in this scenario. Focus on door/hatch inputs, wiring, and lock control outputs instead. [Elektroda, Anonymous, post #16699923]
I replaced the driver’s door lock actuator and nothing changed. What next?
That mirrors the thread: a new driver’s lock didn’t fix it. Move on to wiring verification and module input checks rather than repeating actuator swaps. Use a scan tool to confirm latch states. [Elektroda, nikus1984, post #16699765]
What quick checks should I do in the driver’s door grommet?
Pull back the rubber boot and inspect each conductor for breaks or corrosion. Flexed doors often crack lock, latch, and ground wires. Repair any damaged wires and re‑test lock commands. [Elektroda, Ireneo, post #16694554]
What’s an edge case I should not overlook?
A weak ground at the third stop light can silence the hatch latch sensor. The GEM then blocks locking because it “sees” an open hatch. Restoring the ground resolves the logic block. [Elektroda, Anonymous, post #16699807]
Why do the indicators flash once but nothing locks?
That single flash acknowledges the lock request, but the system won’t drive the locks if any closure isn’t confirmed. The one‑flash symptom is part of the original case. [Elektroda, nikus1984, post #16687632]
What is the GEM module in this context?
It’s the body control that validates whether all doors and the hatch are latched before allowing lock actuation. If any input reads unlatched, it inhibits locking. [Elektroda, Anonymous, post #16699807]
I’m waiting two weeks for an electrician—what’s one high‑impact check now?
Fix or rule out the tailgate’s third brake light ground. A shared ground fault can block the GEM’s hatch‑latched input and prevent locking altogether. [Elektroda, Anonymous, post #16699807]
Indicators didn’t blink on my Mazda after locking—could it still lock?
Yes. A user reported bolts locking with the trunk closed but no direction‑indicator blink on a Mazda 6. Logic and feedback can differ by model. [Elektroda, Anonymous, post #16699916]
