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Home / DTC / B1A02 — KOS key3 internal error

B1A02 — KOS key3 internal error

Detailed page for trouble code B1A02.

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B1A02

MITSUBISHI B — Body

KOS key3 internal error

Brand: MITSUBISHI
Type: B — Body
AI status
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Page language: EN

Causes

  • Faulty or corrupted key transponder/fob (key3)
  • Failure inside the KOS control module (internal processor or memory)
  • Intermittent or lost power/ground to the KOS module
  • Damaged connector, wiring, or antenna/coils used for key detection
  • Communication error on CAN/LIN network between KOS and other modules
  • Software fault or required module reprogramming

Symptoms

  • Keyless entry, remote start or push-button start not recognizing the affected key
  • Engine does not crank or immobilizer prevents start with key3
  • Warning or security lamp illuminated on dash
  • Inconsistent detection of the key (works sometimes)
  • Other door/lock/immobilizer functions operate normally with other keys

What to check

  • Scan with an OEM-level diagnostic tool and record freeze frame and related codes
  • Verify presence/absence of other related DTCs (CAN, BCM, immobilizer codes)
  • Inspect key fob: test buttons and measure key battery voltage
  • Check KOS module supply voltage and ground at connector with key present
  • Inspect wiring and connectors between antennas, KOS module and BCM for corrosion or damage
  • Check fuses related to KOS/keyless and body control systems

Signal parameters

  • Key fob RF transmission (expected frequency; key transmits when buttons pressed)
  • Transponder coil/antenna response when key is within reader range
  • KOS module supply voltage: ~12 V with ignition off/on as specified
  • Ground continuity and module chassis ground voltage drop under load
  • CAN/LIN diagnostic messages: presence of regular heartbeat and key ID frames
  • Error/status flags inside KOS module memory (via OEM scanner)

Diagnostic algorithm

  1. Retrieve B1A02 and any other stored DTCs with an OEM scanner. Note freeze-frame and occurrence count.
  2. Verify the issue by attempting to start or perform key recognition with key3 and with a known-good key. Confirm if fault is key-specific.
  3. Inspect key3 (battery, corrosion, physical damage) and replace battery if low. Re-test.
  4. Visually inspect KOS module connectors, antenna amplifiers and wiring for damage, corrosion or water ingress. Repair as needed.
  5. Measure KOS module power and ground at the harness with key present; compare to spec. Repair power/ground faults before further testing.
  6. Use OEM tool to view key registration table and attempt to re-register/reinitialize key3 per factory procedure. Re-test after registration.
  7. Check CAN/LIN bus health and verify communication between KOS and BCM/ECM. Address any communication DTCs first.
  8. If intermittent or unexplained internal errors remain, attempt a module reset or software update/flash using OEM procedures.
  9. If module continues to report internal error after software reflash and wiring/power/comms are confirmed good, consider replacement of the KOS module (follow pairing and immobilizer programming procedures when replacing).
  10. After repair, clear codes and perform full functional test with all keys; confirm no recurrence and verify no related codes present.

Likely causes

  • Key battery weak or dead for the affected key
  • Physical damage or water ingress in key fob or KOS module
  • Loose connector or corroded pins at the KOS unit or antenna amplifier
  • Blown fuse or poor chassis ground feeding the KOS circuit
  • Stored internal error/firmware glitch requiring reset or reflash

Fault status

⚠️ Status
KOS key3 internal error — internal fault detected in the Keyless Operation System referencing key slot/key ID 3. May indicate key, module hardware, power/ground, wiring, or communication issue. Further diagnostics required.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0 - 3.0 hours

Similar codes

B1A00 B1A01 B1A03 B1A04 U0121 U0140
406

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