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Home / DTC / B1A12 — KOS key3 low battery

B1A12 — KOS key3 low battery

Detailed page for trouble code B1A12.

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Code

B1A12

MITSUBISHI B — Body

KOS key3 low battery

Brand: MITSUBISHI
Type: B — Body
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Depleted/weak key fob battery (key 3)
  • Poor or corroded battery contact inside the key fob
  • Incorrect battery type or wrong installation orientation
  • Damaged key fob electronics or antenna
  • Intermittent RF transmission due to internal fault or water ingress
  • Receiver/antenna wiring fault or poor ground at vehicle KOS module

Symptoms

  • Warning message or lamp indicating key battery low (instrument cluster)
  • Reduced remote locking/unlocking range
  • Intermittent failure to detect key for push‑button start or automatic unlocking
  • Inability to start when key is not recognized or must hold key close to button
  • Multiple attempts required to lock/unlock or start vehicle

What to check

  • Retrieve stored fault code(s) and freeze frame using an appropriate diagnostic scanner capable of reading manufacturer codes
  • Verify which key (key 3) is indicated by the code and test other known-good keys
  • Visually inspect the key fob for damage, corrosion, or water ingress
  • Measure the key fob battery voltage with a multimeter (remove battery per manufacturer instructions)
  • Confirm transmitter range by performing locking/unlocking tests at varying distances
  • Inspect KOS receiver antenna and wiring at the vehicle for corrosion, loose connectors or damage

Signal parameters

  • Key battery voltage (V) — typical CR2032 nominal ~3.0 V; considered low below ~2.5 V (threshold varies by design)
  • Remote transmit frequency — typically 315 MHz or 433 MHz depending on market (verify vehicle spec)
  • Detected key count/status reported by KOS module (which key IDs are present)
  • Remote operation range (meters) — significantly reduced range suggests low battery or transmitter fault
  • Module error/communication status on KOS/immobilizer bus

Diagnostic algorithm

  1. Connect diagnostic tool and read/record code B1A12 and any related KOS/immobilizer codes (note freeze-frame data)
  2. Confirm the affected key (key 3). If possible, test function with other keys to isolate to a single key or the vehicle receiver
  3. Power down and open the key fob following manufacturer guidance. Visually inspect battery contacts and PCB for corrosion or damage
  4. Measure battery voltage with a digital multimeter. If voltage is below manufacturer recommended level (commonly
  5. Reassemble key and verify remote lock/unlock and proximity detection. If necessary, test at different distances to confirm range improvement
  6. Clear the DTCs with the scanner and attempt normal vehicle start/entry operations. If code does not return, monitor for recurrence over several cycles
  7. If replacing battery does not resolve: try using a known-good spare key. If spare works, replace or repair faulty key. If none of the keys work reliably, inspect vehicle KOS receiver antenna, wiring and connector continuity and grounds
  8. If wiring and keys check good but problem persists, consult manufacturer procedures for key reprogramming/identity re-learn and perform if required. If still unresolved, consider replacement/repair of KOS module or immobilizer unit per service manual

Likely causes

  • Key 3 battery voltage below expected threshold (end-of-life)
  • Corroded battery holder preventing reliable contact
  • Physical damage to key PCB or antenna reducing transmit power
  • Vehicle KOS receiver sensitivity degraded or wiring/connectors loose
  • Key not learned correctly after battery change

Fault status

⚠️ Status
KOS key 3 low battery detected — replace or service key fob battery and verify key recognition.
🟢 Repair difficulty: Easy
⏱️ Diagnostic time: 0.5-1.0 hours

Similar codes

B1A10 B1A11 B1A13 B1790 B2799
406

Browse 406 MITSUBISHI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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